How many prime numbers less than 100 have a units digit of 3?
We list out all the positive integers less than 100 with a units digit of 3:  3, 13, 23, 33, 43, 53, 63, 73, 83, 93.  Of these, only 33, 63, and 93 are not prime.  Thus our answer is $\boxed{7}$.