How many Mortgage Credits runs are possible before affecting score

Generally speaking that is accurate. Multiple inquiries for a mortgage in a short period of time have little or even no impact on credit scores used for mortgage lending.

Here is what FICO has to say about it:

MyFico: “…most credit scores are not affected by multiple inquiries from auto, mortgage or student loan lenders within a short period of time. Typically, these are treated as a single inquiry and will have little impact on the credit score.”