How much blood must be lost during a surgery in order for a blood transfusion to be administered?

Depends: This depends on what your starting blood counts are, what your underlying medical comorbidities(ailments) are, and how your body is responding to blood loss. For some people, it may be many mLs, for others, particularly if they've had ongoing blood loss prior to the surgery, it may be after only a small amount(as in less than 200 mLs).