When it comes to the natural number system, there is no such thing as a last number. Every number, by definition, has a greater counterpart.
Every number in the base-10 number system has a greater number than it. The concept of infinity refers to the idea that there is no such thing as the last or largest number; there is always a number greater than the one picked.
For instance, if someone takes any abstract number N and adds 1 to it, the result is greater than N. N is not the last number that may be picked for any number N.