Hard to say as it depends on the traffic to/from your relay. A public relay with several thousands of active users should get a good CPU and a decent amount of RAM (32 GB or more). That's my experience with nostr-rs-relay at least. Based on nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s's posts, it seems like the strfry relay software performs better though.

If you're only whitelisting your own and some other pubkeys(for example only the pubkeys you follow), you could just do with a cheap VPS as there won't be too much processing power needed.

i.e. depends on factors such as traffic/active users and (possibly) the software.