What is the average number of units needed for a brow lift?

This varies tremendously from person to person depending upon how heavy the brows are, their current location, and your muscle sizes/strength.  Many injectors of wrinkle blockers (e.g., Xeomin, Botox, Dysport) for this indication forget to get the depressor superciliaris muscle, which is an often forgotten brow depressor muscle in addition to the corrugators, procerus, and orbicularlis oculi.  If your injector does not know these muscle names then you need to consider someone else.  I'd say the average amount is in the 36-40 units range for Botox and Xeomin and triple that for Dysport.  The "botox" brow lift only will typically give a few millimeters of lift, so it is not as powerful as a surgical lift.  However, it is a great option for many and can soften your appearance to others, so that you don't have a "harsh" or overly "pensive" looking face.  Careful dosing and placement can allow movement without looking completely frozen.  Find an experienced injector with good reviews and reputation.  This is not a good place to find a groupon special.  The best injectors don't advertise on groupon etc.  Best regards.

 In the forehead, there are different muscles that  raise and lower the eyebrows. Botox works to temporarily paralyze either the depressor functions or elevator functions of the forehead. The amount of units of Botox needed is quite variable depending upon what you are trying to accomplish, and how big your forehead is, along with the strength of the muscles you're trying to paralyze.
 A surgical brow lift accomplishes raising low set eyebrows and surgically softening some of the forehead expression muscles that create wrinkles. For more information and many before-and-after examples of surgical eyebrow lifting, please see the link and the video below