Complex sentence: In the past, there used to be many different gowns for different ranks of undergraduates, but these were abolished in the 20th century, there is generally one or two gowns for a single class of undergraduate.
Split into simpler sentences: In the past, there used to be many different gowns for different ranks of undergraduates, but these were abolished in the 20th century. Currently there are just one or two gowns for a single class of undergraduate.