If relays were built to be small, fast and efficient, using a small, fast, and efficient language, and incorporated a DNS alternative, a million+ relays could be run on SBCs. Combined with a desktop application using nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s's nostrdb, so that 100% uptime isn’t required, it would be damn near unstoppable.