For example, you can license Windows desktop OS by named user or by machine. If you do it by machine, it's cheaper, but if you do it by named user then that user can utilize the license on up to 5 (I think) devices (PCs, tablets, phones, whatever). Generally speaking, licenses by machine are called CALs (client access licenses), and the way you get to a number is by running a report in Microsoft's System Center Configuration Manager (MS SCCM), which is a server platform that helps you manage and distribute software to machines attached to your Active Directory domain(s). However, there is another option for desktop licensing: desktop virtualization using Microsoft Remote Desktop running on a server or VM. In this case, you have to pay for RDP CALs, which cost about 3x what a desktop "Core" CAL does. If you're running Terminal Server (for the RDP clients to access), you have to license that (the server OS + Terminal Server itself) ... however, you can reduce the Server OS license by also licensing Windows Server Datacenter Edition, which lets you pay once per Virtual Host and lets you spin up as many Windows Server VMs on that host as you want.
Anyway, counting all this and figuring out the contract amount is done using these SCCM reports, plus a bunch of manually counted stuff plugged into a complicated Excel workbook with many tabs, formulae and hidden VBA ... and then you start the person-to-person negotiations with Microsoft, because depending whether you're a category A, B, or C enterprise you get different pricing, and you can be in different categories for different products or product lines.
I'd say that each time we have to do a true-up it consumes about 1000 person hours.