As you're aware, the DMG's guidelines for building encounters are designed for parties of three to five characters:
The preceding guidelines assume that you have a party
consisting of three to five adventurers. (page 83)
Many of the published adventures contain recommendations:
- Lost Mine of Phandelver: "...adventure for four to five characters..."
- Hoard of the Dragon Queen: "The ideal party size is four characters."
- Rise of Tiamat: "Four characters is the ideal party size."
- Curse of Strahd: "...for a party of four to six characters..."
- Storm King's Thunder: "adventure for four to six player characters"
Tales from the Yawning Portal contains seven separate adventures, and two of them include recommendations:
- The Sunless Citadel: "It is designed for a party of four or five 1st-level player characters."
- The Hidden Shrine of Tamoachan: "...is designed for a group of four or five 5th-level player characters."
- Adventurers League rules require a table of 3-7 players, so all WotC-published modules will hew to that assumption. Further, rules for adjusting encounter strength will typically put 3-PC and 7-PC parties at very weak or very strong, putting the expectation for "standard" parties at 4-6 PCs.
Princes of the Apocalypse and Out of the Abyss do not appear to contain any such recommendation. From the recommendations provided, it would be reasonable to conclude that, while there is no single number of characters that makes up a "standard" party, parties are generally, just as in the DMG, four or five characters.