Can you order additional Apex Licenses?

I don't know the exact user count in my network of anyconnect users. I want to order say, a 999 user apex license which falls into the group user range of 500-999 apex users. My question is, if I install this, and then find I need to add an additional license, would I then need to buy 1000 more licenses because I've hit my limit within that group (i.e. I would need to move up a group to the 1000-2499 bracket)?

Furthermore, if I orderd 1000 users (which falls into the 1000-2499 group user range) and I needed 1 additional license, could I actually just order 1 license and get this added to my smart account?

I am just confirming what options I have to add more users, so I can include some scalability as the user count increases.