I was prescribed tranxene (clorazepate) 7.5 mg po at bedtime, but i only have tranxene (clorazepate) 3.75 mg tablets. how many tablets do i need to take?

Two, but talk to do: Obviously 3.75 +3.75=7.5 so the answer would be two. However there may have been a breakdown in communication between you and your doctor which led you to be in this situation when you don't know how your doctor wanted you to take your meds. It is also a good idea to talk with the doctor who prescribed the meds to be certain how he or she wanted you to take them.