Complex sentence: The collection in the museum in Chennai (formerly Madras) has a large number of sculptures, which they have classified by four periods of activity starting in the second century BC and stretching to the second century AD.
Split into simpler sentences: The collection in the museum in Chennai (formerly Madras) has a large number of statues. They have divided the statues up by the four periods of activity in creating these sculptures which start in the second century BC and stretches to the second century AD.