I'm usually using 64 bots in coop, that's not too much and not too less.
So in my case I set bot ratio to 100 and bot number to 32. That means all the 32 bots set will go to one team, the other 32 bots scripted in Aidefault.ai will go to the other. So, ratio to 100 and number to the half of the bots you wanna use.
Note that this will only work with bot numbers up to 96, cause 48 is the max bot number you can adjust. If you want more, you'll have to play Sp instead of coop, it balances automatically.