If we're talking number different franchises involved, Project X Zone may take the cake.
I haven't done the math, but might be interesting to do that and compare.
Looking only at playable characters you can have on your team:
Project X Zone: 58 + ?? (You can recruit and play as some bosses, but I haven't counted how many.)
Smash Ultimate: 69 (Including Echo Fighters.)
Project X Zone: 33
Smash Ultimate: 32
If we include bosses you can fight against, it's a total of 98 characters.