That depends. I have two phones (my current GNex and my retired OG Droid) and 2 Nexus 7's (mine and my 6 yr old sons). All of those are registered under my GMail account and any app I buy is available to any of those devices.
My wife has her S3 and her retired HTC Rhyme, and apps she has purchases are available on either of her devices.
As far as apps purchases through the Amazon App store, you can share that account accross multiple phones without having separate Google accounts. They also offer a free app of the day and I have taken full advantage of that when something catches my eye.
I hope this makes sense.