No. A number is followed by the singular, or by o plus the plural:
- wyth ci
- wyth o gŵn
The o plus plural is usually used for 'large' numbers - more than five/eight/ten/... depending on who you speak to! Some people just stick to the 'number + singular', though, especially in the colloquial language, as it saves having to remember what the plural of a word is.
Are plurals really used so infrequently that people don't bother learning them?
Not so much 'don't bother' as 'can't always remember'!
The best thing to do is to learn plurals and genders right from very early on. This means looking them up and noting them down, which is a worthwhile effort IMHO. Time spent on vocab is never wasted.