What are numbers that are not prime called?

Answer 1

There are many different sets of numbers. Assuming you mean only the positive integers and purely the 2 states prime or non-prime, the ones that are not primes are termed composites. One caveat, the number 1 is somewhat unique, being considered neither prime nor composite.

[ However, relaxing this, other numbers like the set of irrationals and reals are mostly not prime as well. This would be numbers like the famous Pi or a number like 2.17]
Composite is a number that is not prime.

Answer 2

Numbers that are not prime are called composite numbers. A composite number is a positive integer greater than one that can be formed by multiplying two smaller positive integers. In other words, composite numbers have more than two factors, unlike prime numbers which only have two factors (1 and the number itself).