I don't see how that should affect the number of party members. There can still be multiple characters, they just can't all battle. Does a novel have more than one character even though it doesn't have a battle system? (what an awful example)
I pity the man who bases the number of party members in his game around the DBS!
Haha, yeah, what an awful example. I view it like this; the player can "control" a party for story purposes, and while the one guy is fighting another one guy in whatever zany CBSes are made, the other party members are fighting their own battles simultaneously. I mean, it doesn't matter much to me how people handle it, but that is an easy out, right?