Well, we first choose 12 positions out of the 70 for the letter $c$: $ \binom{70}{12} $
Then, we choose 30 positions out of the remaining 58 for the letter $b$: $ \binom{58}{30} $
The remaining 28 positions are for the letter $a$: 1
$ |C| = \binom{70}{12} \cdot \binom{58}{30} $