Explaining "their numbers" vs "numbers of them"

Hi! I'm working on editing a paper and came across a sentence that reads "Chinese students continually entered Tohoku Imperial University from around 1920 and the numbers of them increased gradually..." I believe it should be "their numbers increased gradually" instead of "the numbers of them increased gradually," but is that right? And what would be the reason/explanation for this? I'd really appreciate any input anyone could provide!