if i cook 5 steaks at once to be medium, i set the temp to be 139.5 degrees but how long do i cook them???
It depends on how thick each of them is. It can be up to 4 hours without coming to any harm but I think that your temperature is a little to the high side of medium.
As mentioned above, it depends on how thick they are. Also, what kind of steaks are they? Also, 139F should be fine for medium.
You might find this link useful:
To achieve superior SV cooking results plan your cooking times in advance working back from your desired service time.
For even cooking package the steaks individually or side by side, never stacked.
Thickness always determines cooking time.