I just installed audio in my parents new house. 5.1 in the LR, 2 speakers in each...kitchen, dining rm, fr & r porches, master bed. Depends how sophisticated you want it & if you are going to run an A BUS, volume control system.
I got all my speakers at cost, 67% off msrp...so it wasn't expensive at all. I got the speakers for about 57 bucks ea. I used all in ceiling speakers except for the rear porch. Just have to pick up a sub and receiver.