The number one (1) is not a prime number.
Prime number definition:
A positive integer greater than one is called a prime number if its only two positive divisors (factors) are one and itself
For example, 5 is Prime, as its two positive divisors are the number one (one) and the number five (itself)

To further define, the positive divisors (factors) must be two numbers that are not the same, or the result in case of the claim that the 'number one is prime' would be the number one being the singular and only positive divisor, which then invalidates it as a prime.