Well, depends on a lot of factors....room size, efficiency of your speakers, whether a powered sub will be used, how loud do you listen to your movies and music, etc....
At one point, I was considering the ASL AV20's predecessor, the AV8 @ 8 watts/channel. Five of them driving some efficient bookshelf speakers like the Energy C-1 (92db) matched with the Klipsch HT pre/pro (currently $99.00) and a powered sub. Since my system is in a den, and I don't enjoy loud listening sessions, I think that would be more than enough.