I would think dentists or gunmen. When you "bite the bullet," you are facing a problem "head-on." Afterward, you would need a dentist from biting down so hard on the problem. Numbers could come into play when you receive the bill from the dentist.
In the past, people would bite coins to be sure they were actually gold or silver. If you handled a lot of money ... you were "crunching" a lot of coins (or numbers).