If the highest common factor (HCF), also known as the greatest common divisor (GCD), of two numbers is 1, that means the numbers are coprime or relatively prime. This implies that they do not have any common factors other than 1.
For example, the numbers 8 and 9 are coprime. The only factor they share is 1. Other examples include pairs like 15 and 28, 4 and 5, or 21 and 22.
In general, any two numbers that are not divisible by the same number (other than 1) can be considered for this criterion. For instance, you could take prime numbers, since any two distinct prime numbers will always have an HCF of 1. Additionally, if one of the numbers is prime and the other is not a multiple of that prime, their HCF will also be 1.
Thus, there are infinitely many pairs of numbers whose HCF is 1, and they can vary from smaller integers to larger ones, provided they do not share any prime factors.