How many times does an AC electric current with the frequency 50Hz change its direction?

To answer the question we have to first understand it. Frequency of an AC current implies number of complete wave present in a second. 50 hertz frequency means that there are 50 waves present in an interval of one second. The given picture will clear all he doubt,

In the picture there is only one complete wave in a second, hence its frequency is 1 hertz. Also observe that in one wave the current change its direction twice first positive than negative.

Now, if the frequency of the current is 50 hertz, which means that there are 50 such waves present in one second and then the current will change its direction 100 times.