800
99 - from 750 to 849. This is just integers though, there are an infinite number of numbers that can be rounded to 800 if we include decimals.
There are infinitely many such numbers.
0
All numbers can be rounded to the nearest ten.
999 of them.
70
88500
There are infinitely many such numbers.
0
500
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.
600
An infinite number.
530
1000
999 of them.