The two most poplular OS: Linux & Windows
Of-course, it is always better to know beyond these two.
I see paying a fixed amount for a maximum of 2 hours per server or per dedicated server client/colocation client has these advantages:
1. Easy to estimate and easy billing. As simple as pay per server or client, instead of counting the number of tickets every month.
(best of all, less headache)
2. Tech will get to know their client(s) or the server(s) that they provide support for. (specialization)
3. Stable income to tech and increase as they manage more clients/servers. Assume if a tech provides support for a dedicated server client or a colocation client ( for a max of 2 hours per month) for $100 per month. If that tech provide support for 20 clients, then it would be $2000 per month ( stable monthly income) to provide support for only 20 clients ( the 20 clients are dedicated server owners and colocation server owners only, not clients of dedicated server owners or colocation server owners).
As you know, not all 20 clients will use all 2 hours every month, unless they keep messing up their servers or something goes wrong. Up to 2 hours per month per client for 20 clients = less than 40 hours per month for $2000. Hours will increase as tech have more clients to handle.
I think it is better to have per client basis than per server basis, because who knows how many servers will one client have. ( 2 hours limit per client still apply)