Tansy Planting Density

I'm getting ready to order two varieties of Tansy, but I'm not sure how many I need. I'm mainly going to be planting them around the foundation of the house to help keep ants out.

Plant tansy at one foot centers. It makes runners and will soon fill in the space. To help with this and to keep it where you ant it, surround the the area you want it to occupy with a plastic barrier ( the kind used to keep grass out of flower beds) sunk about 20 cm (8") into the ground.