Celery and Leeks

There was a young lad at the plots over the weekend who is trying to start up a business selling plug plants and other items to local allotments (none of us have allotment shops at the moment so this might just work out for him).  Anyway as I liked his enthusiasm I decided I had better buy something off him.  He had Celery and Leeks that were well advanced on anything that I have coming along so I bought a tray of each.  When I got them home I discovered that in each of the six modules in the trays there are about a dozen plants - should I leave these as they are or should I divide them up and pot them into individual pots?