Hyaluronidase under eyes risks and right amount of units to avoid problem?

That depends on different factors. I would assume that the filler you are referring to is intra-orbital on top of bones or subdermal in tear troughs . 
It depends on location , amount of filler used, and amount of correction needed.
Starting between 20-50 Units on each side is reasonable. If needed more sessions can be done. 
It is the same when filling this area. You'd rather under fill or under dissolve than over doing it.
Start low and go slow applies perfectly for this area.
Hope this helps.
Chadi Dahabra 

Of course the best way is not to use any hyaluronidase at all by not using hyaluronic acid fillers in the lower eyelid area whatsoever. But if the patient has had one of the hyaluronic acid fillers placed into the lower eyelid, then I recommend starting on the lower end of hyaluronic acid use. I generally tell the patients that they may need two or three treatments but it is a lot safer this way. I start with 20 units of hyaluronidase per side and revaluate the patient two weeks later. Most of the time even a single treatment appears to be sufficient. Good luck from New York City plastic surgery