If in doubt, 3 plants per metre is the ideal spacing for almost all hedge plants. Any plants that are an exception to this are noted on their product page.
Almost all bareroot hedging is planted at 3 plants per metre, 33cms apart. The only exception is Common Box, Buxus sempervirens, which is usually planted at 5 plants per metre, 20cms apart.
Double rows: Country hedging plants are sometimes planted in a double row to make them extra stockproof. A double row hedge requires 5 plants per metre. To make a double row, you just plant 2 single rows in parallel, with 40-50cms between the rows. Make the rows staggered by placing the starting point for one row 16.5 cms further along than the other row. A double row needs 5 plants per metre.
Most pot-grown hedging is also planted at 3 plants per metre. There are a few exceptions:
How important is the spacing between the plants in your hedge?
In general, 3 plants per metre is the ideal spacing: this will give you a thick hedge quickly. If you planted any closer together, you wouldn't really get any benefit and the plants would compete with their neighbours too much.
On the other hand, if you spaced your hedge plants further apart, it would take a bit longer to get a really thick hedge and if the plants where far enough apart, a person would be able to push through them, even when the hedge was mature. So if you aren't in a hurry and security isn't an issue, you can plant your hedging at a wider spacing and save some money.