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.
1000.
1000
910.
1000
901.
It is: 1,000,000-1 = 999,999
That number in word form is: one hundred quintillion.
If you mean Roman numeral then it is M = 1000
999 + 1 = 1000 Therefore, 999 comes before 1000.
The number that follows quintillion is "quintillion and one".The order of 1000 that follows quintillion is "sextillion".
1000 Trillion = 1 Quadrillion. Therefore the next number would be 1 Quadrillion and 1, or 1 000 000 000 000 001.
M = 1000 C = 100 L = 50 X = 10 I = 1 If a smaller number comes before a bigger number it is subtracted → MCMLXII = 1000 - 100 + 1000 + 50 + 10 + 1 + 1 = 1962