As I read from the release notes, the Master AP and Standby master AP can't be Mesh AP's or Ethernet-linked Mesh AP's.
Taking that into account, how many AP's minimum is needed to take advantage of mesh at all? Eth-linked Master, Eth-linked Backup Master, Eth-linked Mesh, and Mesh? Four AP's to avoid only one cabled link? Is that correct?
You could have only 1 Master (Root AP) and one Mesh AP. Only downside is, then you wouldn't have a Standby Master in that case, so no redundancy; if the Master goes down the whole network (only 2 APs in this case) would be down.
You can also have - and I think this would be the recommended deployment - 1 Master, 1 Standby (both Root APs with wired Ethernet connection) and 1~23 Mesh APs.
Mesh and Smart Redundancy in Unleashed works as follows
1. For an AP to be a Master or Standby it MUST have Ethernet cable connected 2. Both Master and Standby AP or any Member AP can become a Root AP as long as they have Ethernet cable connected
If you have only two Unleashed APs in your network you can still create a Mesh network. In other words, AP with Ethernet connected will act as Master + Root AP. The AP without Ethernet cable will be a Mesh AP (piggy backing on to Master/Root AP). However, since Mesh AP has "no Ethernet" it can't be a Standby AP.