Is their a limit on the number of mons you can mark "favorite"?

I don't think there's a limit. I've favorite more than 30 before and hasn't given me a limit. I guess it's worth testing out.

40

For what reason do ya'll do favorites? We were doing the ones we liked best....
Now I favorite all the ones that i have enough candy to mass evolve except pidgey and ratattata. So when I get 40-60 "favorites" I use a lucky egg to mass evolve. When those are all evolved I go to Pidgey and then ratts until time is up.