Incidences vs incidents

Incidence refers to the frequency of something happening.

Incidence noun : the number of times something happens or develops : the rate at which something occurs

An incident is an event.

Incident noun an unexpected and usually unpleasant thing that happens

Your usage is on the right path.

Usage A

I will analyze the number of incidents of traffic accidents which occurred last year.

This is correct, but sounds a bit clumsy. You could just say:

I will analyze the number of traffic accidents which occured last year.

or

I will analyze the number of traffic accident incidents that occurred last year.

Usage B

I will analyze the incidences of traffic accidents which occurred last year.

You should use singular incidence.

I will analyze the incidence of traffic accidents which occurred last year.

You are analyzing the frequency of a single metric - traffic accidents.

You would use incidences if you were analysing multiple metrics.

I will analyze the incidences of traffic accidents, plane crashes, and boating accidents, that occurred last year.

Though one seldom sees the plural of "incidence", here is an example of its use:

“The incidence of measles in the U.S. was ‘x’ people per thousand in 2015. The incidence of mumps was ‘y’ people per thousand in 2015. These incidences were much worse than they should have been.”