21 inquiries is almost certainly graded an 'F' (by just about any scale) because I'm sure that most people with 21 inquiries are people with bad credit who can't get approved for anything, but keep applying anyways (thus digging themselves in deeper).

Sure, you "can be" successful with a ton of pulls, but once you start hitting double digits, it's generally time to cool off. But the best indicator is the "too many inquiries" rejection. Haven't gotten it yet? Good news, you're batting at 100% and are good to go. Got rejected for too many inqs? Probably time to stop applying for a bit.