19.
{04,14, 24, 34, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 54, 64, 74, 84, 94}
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There are 21 two-digit prime numbers.
There are 17 of them.
7 two-digit numbers.
There is only one such number.
There are 90 two-digit numbers... starting with 10 and ending with 99.