In the natural number system, there is no such thing as the last number. Every number has a number greater than it by definition.
Every number in the base-10 number system has a greater number than itself. The concept of infinity refers to the notion that there is no final or greatest number; there is always a number that is greater than the one chosen. For instance, if a person choose any abstract number N and adds 1, the resulting sum is greater than N. For each given integer N, N is not the last possible number.