600
-- If you include decimals and fractions, then there are an infinite number of them.
-- Regarding only whole numbers, all of them from 550 to 649 become 600 when
rounded to the nearest hundred. That's 100 integers all together, but you can
drop out the '600' itself, leaving 99 numbers.
Infinitely many. Any number in the interval [550, 650] will be rounded to 600: for example, 583.702
500
It depends on whether you are rounding to the nearest integer, nearest 10, nearest 50 etc.
Infinitely many, since between any two numbers, there are always additional numbers. The exact range of numbers that can be rounded to 3000 depend on how you are rounding - specifically, if you are rounding to the nearest integer, the nearest 10, the nearest 100, or the nearest 1000.
1000
Unfortunately integers ending in 0s are ambiguous. If 40300 is rounded to the nearest unit, there are five significant figures, if 40300 is rounded to the nearest ten, there are four significant figures and if 40300 is rounded to the nearest hundred, there are three significant figures.
88500
There are infinitely many such numbers.
0
500
800
21079000000 rounded to the nearest hundred thousand is 21079000000
There aren't any numbers that round to 350 to the nearest hundred.
All numbers can be rounded to the nearest ten.
An infinite number.
530
1000
999 of them.