For personal use, you can install apps on all Macs you own or control. There is no limitation in the number of Macs. Presumably installing an app for a friend could also be a legal use – since you control that Mac as demonstrated by logging in.

Even when there was a number limit, it never crossed the PC–iPod barrier. When music still had DRM you could play it on five PCs/Macs and an unlimited number of iPods.