I assume you are talking about your Reg B adverse action notice, correct? If you tell the client that the number of inquiries on their credit report was a reason their application was denied, then your underwriting standards would address how many is too many and how far back you should count. There is no universal number or industry standard for denying an applicant for that reason.
If you are talking about the reason listed on the credit report itself, most bureaus generally measure the number of inquiries in the last 6 months. I don't know what their definition of too many inquiries is though (you would have to call the bureau you use).
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.