Tricky question, that one. How many babies do you wish upon yourself? If you want to collect eggs, hatch them, and raise catfish, you might want 15. But, if you simply want to replace fish for your population, 0 is best (Buy a few to replace yours..they're cheap). So, somewhere in between lies your answer. I haven't been a proponent of encouraging catfish to reproduce, simply because you lose control of numbers...then catfish are notorious for outgrowing their environment. EVERY call I receive this time of year related to fish kills is related to catfish, overfeeding, overpopulation circumstances.

Teach a man to grow fish...
He can teach to catch fish...