There is no hard and fast rule for estimating the number of ground cover plants needed for a particular location. The number of plants depends on the desired result and the funds available. Naturally, the closer you space the plants, the faster they will cover the ground.
Usually, such plants as English ivy (Hedera helix), Japanese spurge (Pachysandra terminalis), and periwinkle (Vinca) are planted on 1-foot centers; cranberry cotoneaster (Cotoneaster apiculatus), juniper (Juniperus), and wintercreeper (Euonymus) on 3-foot centers. Trailing roses, Virginia creeper (Parthenocissus quinquefolia), and other large-scale ground covers are often spaced no closer than 5 feet apart.
Some woody plants, such as junipers, will eventually mound up if they have been planted too close together. If you must space the plants close together to achieve quicker coverage, be prepared to move some at a later time.
In arranging the plants, some gardeners opt for staggered rows, others for straight. On slopes, staggered rows are preferred. They help prevent erosion by not allowing water to run off in a straight line.
When planting on slopes, the soil must be held in place until the plants are established. A mulch alone is sufficient in some cases. When the slope is steep, use jute or a similar netting to hold the mulch in place. Jute is usually available in rolls 4 or 6 feet wide. Unroll the netting from the top of the hill and hold it in place with wooden pegs or wire staples (coat hangers are easily adapted to this use). Plant through the holes in the netting.