I didn't think there was one. How much are you thinking of doing? 20, 200?
Well i know people, real friends, who do like 20 per day just randomly.
Give it a test on a few accounts maybe?
For example, use an account to poke 10 times, another account to poke 20 times, and keep going up to like 50, 100 etc and see if the accounts get banned. The ones that do, dont go that high again, the ones that dont, you know its safe to do that amount.
...How do you plan to make money by poking people?
I personally find it annoying. If you poke me, I may delete you.