When buying ground cover, look at the plant’s predicted spread. If the spread is 2 feet at maturity, then you’ll want to plant every 2 feet. So if you’re planting in a 100 square foot area, you’d need 50 plants.  If you want to fill the space quickly, you can choose to plant ground cover closer together. Generally, ground cover grows well when planted 12-24 inches apart.  Also, consider individual growing habits. Certain ground covers like sweet woodruff will fill a space fast, so they may not need to be packed as tightly, while other ground covers like bloody cranesbill have a wide spread but grow slowly, so you may opt to plant them closer together.