Area of crown is much  bigger than the front. To transplant in this area. To achieve the same density. need much more hair than the front.
The  balding process of the crown is never stop  expanding . After transplant the  bald area in the crown might still  get bigger.
So transplant just enough to improve the look but not to get maximum density.Better keep the limited  doner hair  for the front.
Crown transplant need to follow natural whorl pattern.

Natural whorl pattern:

1. Single whorl  whorl, whorl
2. Double  opposite  whorl   , 
3. Double  parallel    whorl   ,

Hair pattern of whorl  look like  number . If anyone have triple whorl . The scalp will look like  on his head.!!!