OpenDNS when I left was running I think 12 locations with ~4 machines each for approximately a million queries a second. But we did a lot more work than Cloudflare (loading each users preferences into memory, making a decision of how to answer based on them, logging stats for the user, etc) and had plenty of overhead.

DNS isn't hard to serve at that scale compute or bandwidth wise, your problems are more administrative and maintaining enough overhead for misbehaving clients to accidently DoS you.