For events, it's generally better to have more characters.

The stat boosts from affection are definitely not something to overlook... If you're not going to ignore affection, then having more characters is better. Based on not using the training hall, a character with fully recovered mood can fight for about 50 consecutive battles. Once you've finished 50 battles, you can switch characters and continue. It would be more time-efficient to pick a stage with single-wave + single-HP bar enemies and fight for 1 turn, right? However, in the early game when resources are scarce, it's best to focus on leveling up a few main characters. Clear the stages you can quickly as much as possible, and unlock other features that allow for rapid character development. Let your main characters lead the rest! If it's too difficult, you can always borrow another player's character by sacrificing a bit more of your Friendship Points.