I got a complete set of rear brak pads today but there is one pad that has a clip fixed to the back.

Is there supposed to be 2 of these type of pads?

Which outer side does this one with the clip go on or does it matter?

a Pic of it can be seen here rear brake pic

It's the one on the upper right

 

That is just the pad wear indicator, it causes the squeal when your brakes get low because it contacts the rotor. Should be 2 there one for each side and it is usually mounted on the inner part of the caliper but I doubt if it makes a difference. You will be fine with just one.