Find the missing number?

The missing number is 19.

The numbers in the sequence are all prime numbers, except for the missing number. The missing number must be a prime number that is greater than 17 and less than 23. The only prime number that meets these criteria is 19.

Here is a list of all the prime numbers less than 23:

2, 3, 5, 7, 11, 13, 17, 19

As you can see, 19 is the only prime number that is greater than 17 and less than 23. Therefore, the missing number is 19.