Depends how wide they will get where you live. Its width estimate ranges from 3-5 feet. Since your growing season in Zone 5 is short, I would lean towards an estimate of 3' or fractionally more per plant. Then to make a hedge, perhaps space them out something like 2'/2.5'/3'. I have seen many pictures of LL in a "small" hedge but I do not remember just how far apart they were on those pictures. Try not to get them too close so airflow helps dry out the leaves and minimizes powdery mildew and other fungal infections. If I had to choose, I would go with 2.5 feet.

Did you end up doing your hedge? How does it look?

IME Little Lime will get closer to 5’ in the north, so I would plant closer to 3.5’.

3-3.5 ft