That will be a beautiful way to grow them Jo Ann.
The amount of plants would be determined by which ones you choose. Some of the semps can get quite large.
I could see a lovely Sempervivum 'Slate' in the mix, and maybe the bright colored Sedum 'Lemon Curl' which keeps that amazing bright yellow color throughout the growing season, if in full sun.
What type of colors do you want to go with?
