Subscription is just as it sounds, you subscribe for a certain period of time(1,2,3 yrs). Support is bundled into that subscription so you get the upgrades/updates. Perpetual is just the opposite, you buy that license and you own it. Support is an additional piece that you will have to continue to renew to get the updates/upgrades.
As for the device count, everything I have read has pointed to EACH memeber of the VC needs to be counted as a device. Not one license for the whole stack.
0) Licensing just changed. The whole "perpetual vs. annual" option is gone. Standard and Advanced are gone. This is actually a good thing. Juniper took the opportunity to drop licensing prices while they went back to an a la carte model.
2) Software updates are part of your annual maintenance. You'd have maintenance on your "platform", and then on each application that you run on that platform.
3) Each physical device is a device. An SRX cluster counts as two devices. A VC stack of 10 counts as 10 devices. This is actually better than NSM: NSM counted a firewall cluster as three devices.