The current version supports down to macOS 10.5 but AVFoundation video encoding/decoding requires 10.7+.
As for installing on multiple computers, this is written at the bottom of the "Purchase" page:
Quote:
The Fine Print
If you own multiple computers, you may install the same license key on all of them, as long as you only use REAPER on one computer at a time.
|
As for "how" there is nothing special to do, you just install it the same way on all computers.