Numbers are followed by singular nouns in Welsh, e.g.
deg darlith arall, tair neuadd breswyl.

Remember to use the feminine forms dwy, tair, pedair if the noun after the number is feminine.

The plural form of the noun can be used with the following pattern:
number + o + plural noun which takes a soft mutation, e.g.
pum deg o dudalennau
ugain o fyfyrwyr.