Unlike most tabletop games, D&D doesn’t have a limit on the number of players you can have in a single group. This kind of decision really comes down to the individual discretion of the DM and the players, as striking a nice balance of players makes for a better experience for everyone involved.
Having too many players can result in an absolute mess where the DM can’t get a single word in edgewise, but not having enough can leave the party dynamic feeling severely lacking. As a rule-of-thumb, the best D&D parties tend to consist of around four to six players, plus the DM. This gives people enough to work with, whilst preventing the aforementioned roleplaying chaos.
Who cares in a video game....?
The "design for 4" is a very cheap arguments. TT =/= Video game.
Nearly all campaign are designed for 4-6 players. Not 4 "but you can 6 if you really wish".