VPS sizing starts with the application rather than a generic visitor count. A lightweight API, a WordPress store, a database server and a container platform can have very different CPU and memory patterns even with similar traffic.
What to consider
Monitor peak memory consumption, CPU saturation, database cache requirements and concurrent processes. Leave reasonable headroom for updates, traffic spikes and background tasks so the server does not run permanently at its limits.
It is usually safer to start with measured requirements and a clear upgrade path than to overbuy resources. Cloud VPS platforms make it easier to increase capacity as demand grows, provided the application architecture can use the added resources effectively.