Find the missing number?

23
16
27
19

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.