Consider the ring starting to rotate from its initial position where it is perpendicular to the magnetic field. Thus initially maximum flux is passing through the loop. When it rotates, the flux passing through the loop starts to decrease. When it becomes parallel to the magnetic field, the flux becomes zero and then starts to increase. Here the direction of the induced emf remains the same. Then as it rotates further, it keeps on increasing, reaches maximum. Here the direction of induced emf changes. According to lenz law, the induced emf tends to oppose the flux when it was increasing and now since it is decreasing, it tends to increase it.
Similarly when it reaches its initial position it changes the direction
so, hence two times a revolution
