Minimum spacing is 18", but if you put them closer together or to their neighbors, the police will not come and haul you away. You'll just have to deal with over-crowding sooner.

If you plant 2, with 18" between them and the neighboring shrubs, you'll be left with 12" between the grasses...not enough. Within 2-3 years, they'll look like a jumble.

Also, keep in mind the shrubs will be expanding their size and space, and the longer you allow a crowded situation to endure, the more you'll disturb the root systems of the shrubs when you move the grasses.

I'd go with one KF, smack dab in the middle.

what about the base? How big does that get? The tall grass is terific but I also need to fill the ground space from right to left, bewteen the shrubs

There isn't a lot of difference between the base and the top of the Feather Reed grasses, as you can see from Google Images.

Here is a link that might be useful: 'Karl Forster' Images

I'm going with the 1

Thanks Donn!