[Historical context] Why "6 to 8 medium/hard encounters" meme is obsolete
The DMG, as well as the Basic Set, contains some self-contradictory guidance on adventuring days. There's a little section which reads:
The thing is, the table that they give doesn't actually match the language in that first paragraph about "six to eight medium or hard encounters in a day." I think most people don't notice this, and even those who do notice it don't usually know why the discrepancy exists. I don't work for WotC, but I can explain the discrepancy by pointing to 5E's historical documents.The Adventuring Day
Assuming typical adventuring conditions and averageluck, most adventuring parties can handle about six to eight medium or hard encounters in a day. If the adventure has more easy encounters, the adventurers can get through more. If it has more deadly encounters, they can handle fewer.
In the same way you figure out the difficulty of an encounter, you can use the XP values of monsters and other opponents in an adventure as a guideline for how far the party is likely to progress.
For each character in the party, use the Adventuring Day XP table to estimate how much XP that character is expected to earn in a day. Add together the values of all party members to get a total for the party's adventuring day. This provides a rough estimate of the adjusted XP value for encounters the party can handle before the characters will need to take a long rest.
*SNIP TABLE*
The "6-8" meme made sense before they revised the difficulty guidelines, back around Basic 0.2. Back then, the breakpoints were ceilings, not floors, so what today is an easy/medium encounter would have been a medium/hard encounter back then. If you do the math using those guidelines, you'll find that you actually can fit 6-8 encounters in.
Unfortunately, when they updated the difficulty guidelines and then printed them in the DMG, they did not update the accompanying text blurb saying that "most adventuring parties can handle about six to eight medium or hard encounters in a day," even though they had changed the definition of "medium or hard encounter."
Concrete example: if you look at Basic 0.1 pages 56-58, there's a Hard encounter given as an example encounter between four PCs (three level 3, one level 2) and four hobgoblins. That consumes 800 out of the 4200 XP budget for the day (3*1200 + 600, per table on page 58), leaving 3400 XP left. If you distribute those 3400 XP evently between six other encounters for a total of seven encounters, that gives you one Hard encounter (hobgoblins, 800 XP) and six more barely-Hard encounters (whatever else, 566 XP). That's because a Medium encounter can be at most 550 XP (3*150 + 100) and a Hard encounter can be at most 825 XP (3 * 225 + 150), according to the table on page 56. So we see that "six to eight medium or hard encounters in a day" held, back then.
Notice that Deadly is "up to 1400 XP" in contrast with today's "at least 1400 XP." Note also that back then there was such a thing as "Deadlier than Deadly" difficulty, which Kobold.com used to call "Ludicrous" difficulty.(Basic 0.1 page 57)
Example: Encounter Difficulty
You've designed an encounter for four player characters andwant to estimate how difficult it's going to be. Three of thefour players have 3rd-level characters and one has a characterat 2nd level (due to missing a session).First, note the XP values that define the four categoriesof difficulty. For each difficulty category on the EncounterDifficulty XP per Character table, you'll find the number fora 3rd-level character and multiply it by three (for the three3rd-level characters), then add the number for a 2nd-levelcharacter.
That gives you the following numbers:• Easy: up to 375 XP• Medium: up to 550 XP• Hard: up to 1,050 XP• Deadly: up to 1,400 XP
Now you look at the encounter you've designed, a fightwith four hobgoblins. Each hobgoblin has an XP value of100, so the total XP is 400. Since there are four hobgoblins,you double the XP value of the encounter; the encounter'sXP value, for the purposes of figuring out its difficulty, is800 XP. That makes this encounter tougher than a mediumencounter, but not higher than the hard threshold—so it's ahard encounter.
If you build a later encounter with four bugbears, withan XP value of 200 XP each, you'd end up with a total valueof 1,600 XP for the encounter. That number is above thethreshold of deadly encounters, meaning it's probably toohard for your characters to handle. If you adjust it down tothree bugbears, your total is 1,200 XP—still deadly, but atleast the adventurers have a fighting chance. Two bugbearsis probably a better encounter for this party: you multiply the base XP value of 400 by only 1.5 for a pair of monsters, giving you 600 XP—slightly easier than the hobgoblin fight.
Contrast that with today's DMG rules. (I'm AFB so I'll refer to Basic 0.5 instead here: http://media.wizards.com/2016/downloads/DND/DMBasicRulesV05.pdf but they are the same rules.) Now the example Hard fight is of a bugbear and three hobgoblins against the same party of three level 3s and a level 2. Due to the addition of a bugbear, it's 1000 XP, which crosses the new Hard threshold of 825 XP. The adventuring day budget hasn't changed, so we've still got 4200 XP total to spend, and 3200 XP to split between six encounters. That gives us 533 XP per encounter, which according to the new difficulty table on page 56 means each encounter is Easy (just shy of the 550 XP threshold for Medium).
The upshot is that whereas Basic 0.1 would have given you seven Hard encounters in a day, Basic 0.5 or the rules printed in the DMG would give you one Hard and six Easy encounters. Both versions preface the "Adventuring Day" XP table on page 57 with a text blurb stating that "Assuming typical adventuring conditions and average luck, most adventuring parties can handle about six to eight medium or hard encounters in a day", but that's a holdover from Basic 0.1. In 5E as the DMG actually published, that statement is no longer compatible with the table it's introducing.
In both cases, there never was any expectation that you have six to eight encounters a day. The expectation was that you don't exceed your adventuring day budget and accidentally TPK the party. 5E's design parameters are built to handle two or three hard/deadly encounters per day just as readily as six to eight easy/medium encounters.
IMO the game is at its best when the PCs are outnumbered and outgunned but not outthought; I like pitting e.g. four 9th level PCs are up against six CR 6 Chasmes and a couple of CR 17 Goristros. DMG guidelines tell me that that encounter is ludicrously difficult (124,500 XP when the Deadly threshold is 9600 XP) but my experience tells me it is about right for a couple of hours of fun. I'd love to be one of the PCs in that party, especially when they collect all the DMG-generated treasure associated with such monsters.
TL;DR 5E guidelines recommend a couple of deadly, a small number of medium-hard encounters or as many as six to eight easy-medium encounters in a day. If you do the math, they don't actually recommend six to eight medium/hard encounters per day. The reason people sometimes think otherwise is due to sloppy editing of the DMG and the Basic Rules, neglecting to update some fluff text when the rule guidelines were updated, somewhere around Basic 0.2. What used to be "hard" encounters back then are now "medium," so it's actually recommending six to eight easy/medium encounters per day or the equivalent in fewer, harder encounters.
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