Start with your actual workload
Player slots alone do not determine the resources your server needs. Framework choice, scripts, maps, vehicles and database activity can have a larger impact than the maximum player count.
Prioritise CPU performance
FiveM benefits from strong single-core CPU performance. A smaller plan on a fast processor can outperform a larger-looking package on older hardware.
Leave room to grow
Choose enough memory for your current resources and keep some headroom for new scripts, events and peak player sessions. Upgrading later should be straightforward.