These is no meaningful answer until you disclose what your sourcing department want to achieve, and what maturity stage your dept. is at.
I see one IT company with 1500 US employee and $800 place only 4 sourcing professional and buyers for the whole US operations. But their list of vendors are as many as several thousands, from cleaning and coffee service, to Oracle, from limo service to KMPG. And each professional covers multiple categories.
In another consumer product company, there are average 10-15 vendors each employee, each of them on several sub-catoeories of one major category.