AFAIK, the same amount of days as a normal full time worker, if they are working the same 20 hours every week of the year. This is as they will only be taking the holidays from the 20 hours. It gets a bit more complicated if they are working a different amount of hours per week, and then it's calculated as a percentage.
I'm sorry if this came out a bit scrambled and if I've got it wrong, somebody please correct me!
Thanks