There is no fixed number, but somewhere between 10,000 to 20,000 troops would be normal. A division - which is half of a corps - consists of three to five brigades, a brigade consists of several battalions, a battalion consists of two to six companies and a company has about 100 men. I'll leave you to do the maths!
Traditionally there were 10 men to a section, 3 sections to a platoon, 3 platoons to a company (= 90 men), 3 companies to a battalion, 3 battalions to a brigade....and so forth, but as Quizmonster rightly says the numbers aren't rigid anymore and you could find 4, 5, or 6 companies in a battalion so his guesstimate sounds about right