You need to determine the square footage to be covered (for a circle, that's pi times the radius). Then you look up one of the handy online charts that specify the number of groundcovers needed for that square footage based on spacing. At 6" for approximately 22 sf, you need about 88 plants. Perhaps a bit less (75?) as there is a tree trunk to be taken into consideration and you probably can't plant very close to it because of roots :-))

And pachy is very tolerant of shade. It is a recommended GC for shady areas.

Thanks so much Gardengal