You can program as many keys as you want but only to those 4 licenses from what I understand.
The keys don't get programmed actually, you are programming the MPEM to recognize the key's specific code.
I have a Candoo, max programmed per license/ski is 8, after that something needs to be deleted to make room in the MPEM memory. For the two skis I own, I program a master key or black lanyard. A backup key or two is handy, keep it as a spare in the vehicle after launching just encase it falls into the water and a fish takes it under.
So with the standard Candoo at $399, 4 skis with 8 keys each in theory.