The quick answer is 100 sq ft/person. But you also need to look for areas that are anticipated to exceed 50 people (i.e. conference rooms) as they become an "assembly occupancy" and the numbers are significantly different.
Per NFPA 101 (Life Safety Code) table 7.3.1.2 "Business Use" is 100 sq ft/person in "gross area".
Per the International Building Code (IBC) 2009 table 1004.1.1 "Business areas" is 100 gross.
