Plants Per Metre In A Single Row Hedge

Spacing a Single Row Hedge at 3 Plants per Metre

The most common spacing for a garden hedge is at 33cm: that's 3 plants per metre in a single row.
When mature, a human will not be able to squeeze through it.

Country hedges that need to be stockproof (i.e. to block horses and cows) and/or qualify for the BN11 Planting New Hedges Grant should be planted at 6 plants per metre in a double row.

In the bird's eye diagram below of a two-metre hedge, the grid represents the minimum possible footprint of the hedge: the mature width will be down to how you clip it, and 33cm would be very narrow for most hedges, but it will do for this example, where we are concerned with the spacing along the length. 

The black dots represent a hedge plant, and the red line marks the end of the first metre.

Correct Single Row Hedge Spacing at 3 per Metre 

The first thing to note is that you (typically) do not put your first & last hedge plant on the 0cm and 100cm mark of each metre that you want covered by your hedge. 
Instead, you put your first plant about 16.5cm (at least) into the first metre, and it will grow to fill the required space.
Likewise, the third plant in the metre will be 16.5cm from the end of the metre.

The diagram below shows what it looks when 3 plants per metre is done incorrectly, with plants on the 0cm and 100cm marks, which leaves you no choice but to space them 50cm apart.

Also, to complete the next metre, you would only need two plants.

Incorrect Single Row Hedge Spacing at 3 per Metre 

There is nothing wrong with spacing hedge plants at 50cm: this is a common spacing for ornamental hedges that serve no security purpose, and thorny hedge plants would still certainly knit together into a secure hedge.
The same principle applies as with a 33cm spacing: you don't put the first and last plant on the 0cm and 100cm marks, but (in this case) on the 25cm and 75cm marks.

Correct Single Row Hedge Spacing at 2 per Metre