How selective are fish (stocked trout) when it comes to breeding?

I've stocked 4 Rainbows and 4 Brookies in a stream near my house and want to know the odds of any of them breeding. Also I have not seen any of them and it is a small stream and I built a dam to hold more water...they could be hiding in a small tunnell or under brush or farther upstream. anyway how many fish do you need to stock for them to breed (on average)?...