A Hi,
Thanks for your question. You can address breast asymmetry with one implant, but it's a bad idea in my opinion. Breast implants don't just alter volume but shape, so you would have one much perkier breast than the other with this approach. Better approaches are to use two differently sized breast implants, or (if you don't want implants) either to reduce the larger breast or fat graft/lipofill the smaller one. My preference is to use the same sized implants wherever possible because even implants of different sizes will age differently, so if I notice some asymmetry in a patient requesting breast augmentation I tend to lipofill or reduce one side, and then use the same implants on each side. I personally wouldn't recommend one implant!
All the best,
Fulvio