Subtract the upstream connection first
A five-port switch commonly leaves four downstream ports; an eight-port switch leaves seven. If the topology uses another switch link, count that link too.
Name every fixed device
Write down the router, NAS, workstation, access point, mesh node, dock, TV, console, recorder, and room feed. Mark which devices actually need 2.5GbE.
A spare port is useful, unused speed is not
One or two spare ports simplify troubleshooting and a known upgrade. Buying eight multi-gig ports for three permanent devices is harder to justify when a gigabit branch can serve printers and hubs.
Avoid casual daisy chains
A second switch adds another power supply, uplink, cable bundle, and failure point. If the first switch is full on day one, eight ports are usually cleaner.