My question is how many native English speakers have a proficiency in
both spoken, and written, Mandarin Chinese, at an advanced enough
level to be qualified for employment which uses Mandarin. (i.e. level
9 or above, on the HSK)

in china you may easily find these highly skilled foreingers 'Lao Wai'
say 5000 people all around of the country; in the rest of world, say,
10,000.

In fact, lots of foreigners can speak Mandarin Chinese fluently, but
few of them can be good at reading and writting Chenese characters
both.
   But, more and more people begain to study Chinese, isn't.
   So, I supposed that there are more than 5000 people all around the world, now.
   Remark: I mean "all around the world" and "now"
   
   Thanks