There is no such thing as a last number when it comes to the natural number system. By definition, every number has a number larger than it.
In the base-10 number system, every number has a number larger than itself. The concept of infinity relates to the idea that there is no last or largest number; there is always a bigger number than another number chosen. For example, if someone chooses any abstract number N, and then adds 1 to it, the resulting sum is larger than N. For any number N chosen, N is not the last number that is possible.