Recessed lighting layout

I am finishing half of my basement. The area is 12'x41' and the ceilings are 7'. It will be used as a family room/playroom. I am going to use 6" cans. I bought 18 of them and am wondering if that will be enough or do I need more? Would 3 rows of 6 make for a decent layout?
ETA: There will be 2 separate circuits and they will be on dimmers.