Should Fajr prayer be repeated if the worshiper finds traces of semen after the prayer, but not before it, and is it counted as a congregational prayer or an individual one?

If you are not certain that what you saw was semen (mani), and you did not feel its emission at the time of prayer, but only saw it afterwards, then you are not required to repeat the prayer. This is because you are not certain of it being semen, and because it is a subsequent doubt that does not invalidate the prayer.