It'll take as much as you give it for caching.. the thing is it's hard to tell because relays don't tend to be very busy unless you really have a huge amount of users.. for nostr21.com I have maybe 100-200 simultaneous connections and it's no where near maxing the machine out (8 gig ram, 4 vcpu vps). It's been running for months but the adoption is a slow ramp. Database on disk is only about 8 gig (running since November 2022), 3 gig when dumped to json.