The maximum number of machines for Pro is currently 3. Thinking ahead to future syncing between devices this number will be more important, and I think it’s too low.
2/3 makes sense for the Essentials version, but Pro should be…Pro. I can imagine someone with a work iMac, work MacBook, home MacBook, home media centre (MAC Mini), and partner’s MacBook being a ‘Pro’ user — that’s 5 devices which is the number I normally see for Pro tiers on other software. Ideally it should just be unlimited for individual/household use.
(I feel less strongly about Essentials, but I do think that should also be unlimited — limiting by devices always seems an artificial division and if you’re going to enforce it then people will run in to problems when the enforcement mechanism doesn’t work correctly)