The number that comes after 1000?

Answer 1

If you mean the whole number that comes after 1000, that is 1001.
If you really mean it the way you wrote it, it is impossible to answer your question.
The reason it is impossible is because between any two real numbers is another real numbers.
For example, 1000.1 comes after 1000. But 1000.01 comes after 1000 but before 1000.1.
But 1000.001 comes after 1000, but before 1000.01.

No matter what number you say comes after 1000, I can find another number that comes even sooner after 1000.