Translation:Un deg pump cath.
Gareth King writes in his Modern Welsh: a comprehensive grammar:
Pump is used where there is no immediately following noun; otherwise pum:
Faint o °blant sy ’da chi erbyn hyn? Pump.
How many children have you got now? Five.
Wi wedi bod yn iste fan hyn am °bum awr.
I’ve been siting here for five horus.
Pum dyn but Pump o °ddynion.
° indicates soft mutation.
So that sounds fairly mandatory to me.
Interestingly, for "6" the matter seems to be slightly different:
The difference between chwe and chwech is the same as that between pum and pump (see previous note), except that some S regions use chwech even with a following noun.