Don't forget the wall behind you.
I'm doing three on the front (one behind each speaker), three on each side wall (two at the first reflection point and one directly to the left and right of the seated positions), three next to eachother directly behind the couch, and four on the ceiling. Beyond that I'll likely flank one on each side of the three on the back wall, but midway between the three and the side walls.
I'm building my own, and since 2" 3# fiberglass come in 6 packs, I have to go with 12 or 18. I could get by with 12, but my room has crazy reverb and echo so I think it's worth the hassle and money to get the extra 6. The only furniture besides the electronics is the couch and two reclining deck chairs.
The good thing is you could always buy more if needed/wanted. You are definitely on the right track.