To keep a constant flow of buds coming in you'll just want to stagger them, so let's say you can stuff 8 into your flowering room, then you could just start flowering like 2 today, put 2 more in in a couple weeks, on and on until you have your 8, and assuming they're the same strain w/ same flowering time then you'd be harvesting every couple weeks. Finding the right balance is gonna take a little more than a hypothetical situation in a post to figure out. Just play around w/ it till you find your zone.
Check some of these links out n get a feel for what others are doing w/ their perpetual set ups...
http://rollitup.org/t/perpetual-growers-thread.844684/
http://www.growweedeasy.com/the-perpetual-harvest
http://www.growery.org/3019/Perpetual-Harvest
As far as how many to actually fit in each area, that can be very much dependent on which strain. For instance I started vegging these two ladies at the same time, same pot size and organic mix, vegged them for the same duration, and put them in the flowering tent at the same time - one ended up taking up almost half of my 4x8 tent, and the the other hardly takes up 1.5 square feet.