I did the math several times, and I agree with s1ncere that you are most likely confusing mg and mL.
Milligram (mg) is a unit of mass/weight. Pills and tablets are, generally, in mg.
Milliliter (mL) is a unit of volume. Vials and ampules are, generally, in mL.
Most liquids in medicine also have a concentration which is expressed as mass/volume (e.g., mg/mL). That is, how many mg are in 1 mL.
So, a vial of T would likely have something like this written on it (all "made up" numbers): 20 mg/mL. This is the vial's concentration. Your prescription likely says something like "10 mg/weekly" and the vial itself is likely to be 1 mL, 5 mL, or 10 mL.
In that scenario, you would draw up 0.5 mL (10 mg)/weekly. How many months that dosage would last you for depends on how many total mL are in the vial. At 0.5 mL a week, you would use 2 mL (40 mg)/monthly. If your vial had 10 mL in it, you had have (roughly) a 5 month supply.
Hopefully that makes some sense. But please do ask your physician or pharmacist for advice before injecting.
