"I want twelve apples."
Translation:Dw i eisiau un deg dau afal.
There is no magic number! People tend to use the singular with smaller numbers and the plural with larger ones, but there is no fixed rule. However...
If you want to use the singular noun then it comes directly after the number:
- saith afal, deg afal, cant afal, ...
If you want to use the plural then it has to be used with the preposition o between the number and the plural noun:
- saith o afalau, deg o afalau, hanner cant o afalau (50), cant o afalau, hanner mil o afalau (500).
There are additional complications if you want to use the compound numbers in the vigesimal system - I don't think that that is covered in this course, and there is enough to learn without adding that into the mix! If you are very keen, though, get hold of a good Welsh textbook such as, say, 'Welsh Rules'.