There is no hard and fast rule regarding the spacing of groundcovers at the time of planting.
The closer the spacing, the faster the bed will fill in. Generally the spacing scheme is based upon the size of the plants, preference, budget, and the location of the bed. The spacing of a bed near a utility area at the rear of a property might be double that for a bed near the entryway. Keep in mind that the longer the bed takes to fill in, the more maintenance in the form of weeding and mulching will be required.
Liberal amounts of weed-free mulch are an essential part of a successful groundcover planting. The mulch helps to deter weeds, retain moisture and prevent heaving over winter.
Groundcovers will spread about 1 inch per month; planted 6 inches apart an area will be carpeted in about 6 months.
An example of how to use the above chart:
Spacing the plants 8" apart on 75 square feet, meet at their intersecting points, 170 plants for the space.
To do your own math: