Traditionally the singular is used for small numbers up to 3, 5, 10 or whatever anybody used to teach as a norm. Over that the form is generally nn + o + plural, although there are other variations, too.. Nowadays the singular is used much more generally, especially in the colloquial registers. Similarly, the decimal counting system is used much more than the traditional vigesimal one, especially by younger people.
- dau ddyn, tri dyn, pum dyn, deg dyn (deg o ddynion), un deg dau dyn (deuddeg dyn, deuddeg o ddynion), un deg pum dyn (pymtheg dyn, pymtheg o ddynion), etc.
Sticking with the decimal-style number + singular is certainly easier as a beginner!