Spacing for Hydrangea Hedge

I'm going for a 'double hedge' effect out front. The lower of the two rows will be Prunus Laurocerasus Otto Luyken, which will be kept pruned to about 2.5ft. Behind that and overhanging the Prunus will be Hydrangea macrophylla which I'll let grow to full height (5-6ft).

The Hydrangeas are different varieties, but all macrophylla.

Question is, how much space do I need to leave in between each Hydrangea? I'm conscious that they can grow almost as wide as high and I don't want any of them to be crowded out, but equally I don't want any gaps. It's a relatively small space alongside the lawn at the front of the house. I have three Hydrangeas, but I'm thinking I may only need two.

Any advice gratefully received

 

If they get crowded could you not prune them back a bit?