by black book do you mean a list of girls who will fuck you on call?
If so I've found around 5 is the optimal number for me. I've had more but I guess I'm quite caring and like to be somewhat involved in the lives of my girls. That and I don't really have any time for them, which is a great contradictory combination haha. You should think about how many nights a week you want to have sex, and assume you don't want it with the same girl all the time. For me I only really have time to have sex with existing girls maybe twice a week. I like to leave room for new girls and chance.
I often find if you pre-plan too much with your existing girls then they block the potential for new girls that day/night because you don't want clashing appointments. (Occasionally, you try to balance the two and end up with none...that's a skill I had to learn...).
If you're talking numbers that are interested but unsure, I'd say about 30 is a good number. It will take a couple weeks to 'process' them...though...and sending the same text to all 30 is not allowed...lol
