I have always found this a bit clunky and inaccurate, I too have wondered about whether these were additive or not.
I'm currently having a problem with licensing on a Mobile VPN that may be a newly created problem since R80.10 Jumbo 154. While investigating I observed this in the GUI:
If you go to Gateways & Servers, Ensure it is on 'general', then the little 'OK' by licence status is a clickable link, this leads to this window displaying exactly what Phil was asking for. Not a number that includes supposition, addition (or not) and deciphering SKUs but an actual count of used and available licences on the gateway.
I bet this is accessible through the 'licensing' dropdown somehow too. What is the CLI to show this? Who knows.
What this clearly shows though, is that our previous (mine included) supposition that the licences are not additive is not correct - they are. This licence is for a 50 and a 5 (the five being the as shipped one.)
What is also new (as far as I can tell) is the fact that the license count hits the secondary at the same time as the primary, I can see why but in this case (and any other where an old SNX licence was upgraded and only one gateway in a cluster has the licence - because it used to reside on the management server and now resides on the gateway will have a problem like this (this is the secondary in this cluster)
This was demonstrably NOT a problem until very recently! The secondary only got involved during a fail-over scenario. I know we need to buy a licence for the HA member, that's not really the point.