Certified GKH Gardening Expert
For Margaret Roberts Lavendula, you can space them about 2-3 feet apart for a hedge. For a normal planting, you would space them 4-6 feet apart and for a hedge, you would want this tighter (hence 2-3 feet).
In order to maintain it as a hedge, you will need to prune regularly. Once a year for a taller style hedge and twice a year for a more compact hedge.