If all that you are concerned about is that the field contains an integer (i.e., not concerned with a range), then add a
CompareValidator with it's
Operator property set to
If there is a specific range of values that are valid (there probably are), then you can use a
RangeValidator, like so:
These will only validate if there is text in the TextBox, so you will need to keep the
RequiredFieldValidator there, too.
As @Mahin said, make sure you check the