99

This number is a composite.

There are 9 ways to express 99 as p + 2q, where p and q are prime. [Hodges]

The 99th prime plus (9+9) is the next prime.

99 = π( prime(9)*prime(9) ). Note that 99 is the only two-digit number with this property. [Firoozbakht]

99!/π(99)! + 1 is prime. [Firoozbakht]