Depends on how thick you want the hedge. If you make a double staggered row then 5-7 per metre (depends how thick the Privet plants are), single hedge then between 3-5 per metre, again depending on how thick you want the hedge to be. If I was doing it I would want it thick, so 7 plants per metre.

Readyhedgeltd.com in Worcestershire, will supply troughs of ready spaced hedging of all sorts, shapes and sizes. The beauty is that it comes in troughs ready for planting straight out.