First, I have never built a pool so I'd like you to know that up front. I seem to remember, generally, #3 on 8in centers, or #4 on 10in centers. Most with 4 sticks of #4 rebar for the bond beam. Of course, it all depends on length, depth, soil conditions, etc. I have a few questions I am about to start on another thread on rebar as well. May give you some more info if I get some responses. Good luck.