I always use them to a minimum. If it takes more memory as you add them? Sure it does. I don't know what is the maximum that is a "correct" number, but depending on the size of your network, the more bots, the more interaction they need to do, the more people using them: the more resources your system will need.
I only have one or 2, like an "official" one, and that's about it. If users want toys so much, they can always load their own bots, like eggdrops or scripts.