Two Macs and One Apple ID (purchases on App Store)
This question can only really be answered on a per app level (The End User License Agreement (EULA) will specify how many machines an application can be installed on), but in general I have a MacBook Pro, My wife has a Macbook Air and we have a media center mac mini and I install all my apps on all of them without an issue, but we share a single Apple ID for all app store purchases, since apple has restrictions on using multiple Apple IDs on a single machine.

Developers seem to have a more sane point of view on mac (excluding Adobe or other graphics packages) than on windows, so I haven't had an issue.