Planning to add about a dozen RCS to my 29 gallon community tank and wondering if it matters how many females vs males I get? I am hoping to breed them, but casually.

I would say every 2 females for every one male. If you hope to breed them in a community you will need LOTS of hiding spots for the shrimplets because any fish will eat them if given the chance to.

If you're just looking to breed casually, and are getting 12, it's almost guaranteed you'll end up with some of each. Unlike many fish, there's not really a set ratio. If you have both and they like the tank, you'll probably have babies, and then, of course, whatever ratio you started with will change.