The numbers 250-259 can be rounded to 300. Also 301-349 can be rounded down to 300. This works if you are rounding by the nearest hundreds place.
3000
The short answer is, infinite. With just integers, There are 498 numbers that can round up to 500, and 498 that can round down. If we include partial numbers, there are infinite, since the numbers can get infinitesimally small.
300
It is 250 rounded to the nearest 100 is 300
300
3000
600 of them. Any number from 300 tp 899 (inclusive), to the nearest 600.
300 cubic feet = 2,244.156 gallons (rounded)
It depends on the degree to which the numbers are being rounded. 300 to the nearest 200, for example.
300
300
300
Ten of them.
Infinitely many.
300
300
999 of them.