It depends on what kind of relay you want to run. Public? Private? Personal? Etc.
Discussion
You can run a private one on your phone or on a Raspberry Pi.
What is private vs personal?
Private meaning only certain people can use it, maybe it's paid, maybe it's for your family, maybe it's for your community group. This could be something like nostr.wine or a WoT relay or something much smaller.
Personal would be just for you, essentially s backup of all of your events and more. This would be something like a #Haven relay.
I guess it's personal since I'm the only one to use it. I'd run a full relay but honestly I don't know the implications of "c" p content nor do I know how to protect against it.
Check out Haven from nostr:nprofile1qqsg4k8377xguyvkvfpw9znu59wfx6er4xvat7u3hljwg3ew94h274gpzdmhxue69uhhwmm59e6hg7r09ehkuef0qy2hwumn8ghj7enjv4h8xtn4w3ux7tn0dejj7qg4waehxw309aex2mrp0yhxgctdw4eju6t09u40ce0y It's what I run for my personal relay.
I'll check it out right now I run my BTC node and nostr relay in the same start9 box I made out of a thin client optiplex. Simple enough for a dummy like me.