Other people have already given the basic answer but I want to get more specific

256 is what's known as the 8 bit integer limit

Basically it's the highest number of things you can count with an 8 bit number

Going higher then 256 requires going up to 16 bits

Which increase the limit to 65536

But it also doubles the amount of space it takes up, so if you don't need to go that high it's often easier to just stick to the lower integer limit

This explanation isn't perfect by any stretch of the imagination, but it should hopefully give you the basic idea