Here's my understanding...
- The MX67 and Z3 count towards the site-to-site VPN number (and if you need to calculate the number of tunnels if you are using dual WAN/internet connections)
- The Windows 10 VPN client counts towards the maximum client VPN tunnels, and the 500 client devices (since they are both a client, and they are establishing a VPN tunnel)
- The recommended number of client devices is the number of devices on the 'inside' of the MX, so pretty much the devices that are listed when you look on the Network-wide -> Clients page of a network
I believe other than the 'Max concurrent site-to-site VPN tunnels' they are all recommendations and your mileage will vary depending upon traffic flows and the other features you have enabled on the device. Any VPN terminations have a high impact on performance due to the encryption/decryption that the processor has to do.
Best approach is to monitor the appliance utilisation, under the organisation -> summary reports -> then select the appropriate appliance network, and when you start seeing utilisation consistently getting towards the 80% mark then I'd start planning the upgrade.