Infinity is not a number, it’s a concept so you can’t perform arithmetic operations on it, such as adding multiplying, etc. There are many “infinities”. For example, there are an infinite number of odd numbers and an infinite number of even numbers. If we add the two sets together to give us the set of natural numbers, there are infinite numbers in the set, not twice infinity! There is an infinity of numbers, including rational, irrational, natural numbers, etc., but it’s not the same infinity as the infinities for each individual set considered separately. They are indefinite, undefined quantities.