There are 2 cases.
1) If number is field of class String then this validation doesn't work at all
2) If number is field of type int then this validation doesn't work also, but besides if I enter not a number then the following exception is thrown:
If the action property is a string then no, validating it as an integer won't work: strings and ints aren't the same things.
If the action property is an int then as far as I know the int validator works, I've been using it successfully for almost every S2 version. If you enter something that isn't a number then you're getting a type conversion exception. Type conversion messages are added to the field's errors via the "conversion" interceptor.