OK, first you have to understand that in some way we are talking about two different things. Number come in two forms, cardinal numbers and ordinal numbers.

The ordinal number deal with value and the order of number, whereas cardinality deals with the size of groups of things. If you have a set of 3 number, the number are 5, 7, and 13. The ordinality of the largest number is 13, but the cardinality of the set is only 3, because there are only 3 things in it.

Now look at how many numbers exist. If one looks at the cardinality of the set of integers, there are infinite integers. Some people might say that if you only look at the odd integers, even though all the integers are both odd and even integers, thus the number of even integer should be less than the number all integers, but in fact, you can show that for every integer there is an even integer that is twice the integer (i.e. Y=2X, where X is an integer, and Y is an even integer, so the set of Ys must be the same size as the set of Xs), so there must be an equal number of integers and even integers.

But now look at the number of real numbers that exist. If one looks at the carnality (size of) the set of integers between 3 and 5, there are three integers, but how many real numbers exist in that set. Even between 0 and 1, there are an infinite number of real numbers. So if one looks at the number of real numbers between all of the infinite number of integer numbers, you find that you have a set which is an infinite times an infinite set.