Parent complaints can be very frequent, depending on where you teach. As far as I know, though, I haven't gotten any complaints this year, other than one child was having problems with another child. One a month might be normal where you are.
If my VP or principal complained, though, I would be more upset. I guess it depends on what the complaint is. I've been corrected on procedures, but I wouldn't really count that as a complaint. I would hope to get only a couple a year, if that.