Hi Doesia..

Well... you can start to flower your plants whenever you want...

If you flower 12 plants that are 12 inches tall.. they will all double in size to 24 inches tall....
1 - 600watt HPS will flower them nicely... if grown properly you should get 1-4 ounes per plant

You could also grow 16 plants very nicely in a perfect 4X4 square..

: )