Inapropriate number usage is very common on Wattpad, but that is understandable since in school, at least my school anyway, it isn't really taught unless you ask about it. The rule of numbers is you only use then when you are either takling about the time (e.g. 15:56) or the number you are saying takes three or more words to say (e.g. 176). There are other exceptions but they don't usually come up and it's usually pretty obvious if you should use the numbers. If the number you are saying takes two words or less to express then you write it out (e.g. 21 would be twenty-one).
The Don'ts of Wattpad
By RosesGardens
A list of writing problems common among many Wattpad writers that I have complied over a year of critiquing. More