Plurals - only two?

Similar problem where masculine nouns undergo an Aspirate Mutation after 'tri' (three) eg:

• 1 car = 1 car
• 2 cars = 2 gar
• 3 cars = 3 char
• 4 cars = 4 car / 5 car...

Usually you add the prefix -au (e.g schools = ysgolau) but in some loan words you just add an s (e.g wikis = wicis). Sometimes a w is added at the end of a word but not much as far as I can recall (sorry I’m using a swear word as an example of this; tits = titw).