when you get the plugs from Prairie Nursery the labels will tell you what the spacing should be. i usually put them a little bit closer than what they reccommend... i think i have spaced some around 8" to 12"... maybe less.

Are you planning mostly grass or a prairie effect with flowering plants too?

If you want room for flowers between the grasses, try spacing at 2 to 3 feet. Indian grass and Big blue stem grow mostly upright but switch grasses are wider.