The 40 students is for all those doing AMES across all specialisms as I understand. I gather Chinese is one of the "bigger" options (along with Arabic) though so I imagine a few.
Looking through a couple of FOIA requests it looks like at least around the last 5 years it's been about 15-25 of those 40 successful offerholders for AMES are for the Chinese programme. That said I imagine that the lectures and classes may have more students accounting for those doing Chinese as a subsidiary language on other AMES programmes and postgraduate students.