It’s in chapter 3 of the DMG, under the header, “The Adventuring Day.”
Assuming typical adventuring conditions and average luck, 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 toprogress.
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.
So, 6 to 8 medium or hard encounters is what the game expects players to be able to handle in one day, and that is the baseline the math is built around. Again, this is not a mandate, you can deviate from this and it’ll be fine. It’s just the assumption around which things are balanced. The fulcrum. And while this isn’t explicitly stated in the DMG, it is evident from an analysis of the math underlying various systems, and from anecdotal experience.