Technically, there is no limit in the number of foreigners employed by a foreign company in China, provided that first they get approved and obtain the license to employ foreigners, and no jobs related to the employment of foreigners could be performed by a Chinese national.  Of course, foreigners employed must comply with all th local laws regarding working in China.

There are locally applied regulations related to registered capital and type of foreign owned business (WFOE, RO, JV etc.). In some areas, businesses with less than 100K USD registered capital can only obtain a work permit for the named legal representative on the business licence. In some areas, that figure is much less. Foreign owned companies are no different to domestically owned businesses in so much as not being able to employ a foreign national unless the position being filled, for whatever reason, cannot be filled by a Chinese national.