Changing number, multiple sims on same network can be put into same iPhone?

HI, I recently finished a 24 month contract with O2 for a iPhone 4s, I upgraded yesterday to a 5s also on O2. I am wanting to keep the same number but I was wondering how to do so. Also I was wondering can you put multiple SIM cards from the same network in an iPhone or will it specifically lock to only 1 sim regardless of another sim being on the same network? Many thanks