As mentioned in Welsh numbers 1 to 10, there are a few points to look out for with Welsh numbers.
One of these is that there are two ways of counting. There is a traditional system and a modern (decimal) system. The traditional system nowadays is more commonly seen in telling the time. It can be seen here in the number ugain 'twenty'. There are two commonly used words for 'twenty'. Ugain comes from the traditional system of counting and dau ddeg from the decimal.
See also numbers 1 to 10, numbers 30 to 1000, and telling the time.