What is your upgrade policy?
Minor updates to the version you purchased are completely free until the next major version becomes available. However, major updates (for instance 1.x to 2.x) require an upgrade fee.
Licenses are personal. Installing your license on multiple Macs is fine, as long as you are the only user of the license. For example, you are allowed to use one license on both your desktop and laptop, but not on all your friends' Macs.