12

This number is a composite.

The concatenation of n!, (n-1)!, ..., 2!, 1! and 0! is prime for n=12, 6, 4, 3 and 2, but not for n=11, 10, 9, 8, 7, or 5 (nor for 13, 14, 15, ... up to at least 61). Note that 2, 3, 4, 6 and 12 are exactly the non-unit divisors of 12. [Hartley]