The models, one at a time
Per user
You pay for each person covered, whatever mix of laptop, phone and accounts they carry. It is the most common model for managed support because it scales with the thing a business actually thinks in: headcount. Hiring someone has a known support cost attached; someone leaving should reduce the bill, and checking that it actually does is a surprisingly good test of your provider's attention. The soft spot is the counting rule: shared mailboxes, service accounts and part-timers all need a written answer before the first invoice, not after.
Per device
The same idea measured in hardware. It fits businesses where devices are shared or outnumber people: shop floors, warehouses, clinics, anywhere a workstation belongs to a shift rather than a person. In a one-laptop-per-person office it is just the per user model with more counting.
Per hour
You pay when something breaks, for the time it takes. Honest for genuinely occasional needs, and you pay nothing in a quiet month. The behavioural cost is real though: hourly billing quietly discourages the early, small call, and IT problems are cheapest exactly when they are small. Nobody is paid to prevent anything under this model, only to repair it.
Blocks of hours
Prepaid bundles of hourly time, usually at a better effective rate. A sensible middle ground for the business that needs help several times a year but not every week. Ask two questions before buying one: do the hours expire, and what happens when the block runs dry in the middle of a problem.
Flat monthly fee
One figure, agreed scope, everything inside it covered. This is what "fixed price IT support" and "monthly IT support plan" usually mean in practice. Predictable for you, and it puts the incentive where you want it: under a flat fee, prevention is the provider's profit margin, so things being set up properly is suddenly in both your interests. The entire model lives or dies on scope. A flat fee with a vague scope is not fixed pricing, it is a recurring argument with a direct debit attached.
Per project
A defined job at a quoted price: a migration, a rebuild, a cleanup. The right model for work with a beginning and an end, and the wrong one for ongoing cover, because the day after the project closes, nobody is responsible for anything. Most businesses end up pairing it with one of the models above rather than choosing between them.