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.
Any number can be rounded to any place value.
600 of them. Any number from 300 tp 899 (inclusive), to the nearest 600.
One thousand of them - to the nearest thousand, Two thousand of them - to the nearest two thousand, Four thousnad - to the nearest four thousand None - to the nearest whole number.
6500 to 749 9999 of them.
We're rounding to the thousands place. How many thousands are in 3,100? Label the lower endpoint.
21079000000 rounded to the nearest hundred thousand is 21079000000
An infinite number.
All numbers can be rounded to the nearest ten.
There are infinitely many such numbers.
999 of them.