Hi
In some cases you need to recognize individuals, such as in capture-recapture analysis for abundance/density estimations, or you might want an index of individuals that are detected (instead of number of detection).
If you need to recognize individuals is better if you have both flanks of the animal photographed, and even better if the animal has natural marks (such as rosettes or stripes like jaguars or tigers), but you can also use scars and general body complexion to help.
You can still make a capture-recapture study with one camera trap, but you have to choose only pictures of one side of the animal to use, and discard all the other pictures of the other side (since you won't know if are the same animal), doing that you decrease the detection probability, because now it would be the product of the likelihood of the animal walking trough the camera and the likelihood of going in the direction that you need.
Using one camera per site is a good setup for a site occupancy study, and you can even estimate abundance with the Royle-Nichols method, if your data fits it.
I hope this helps
Bests
Juan