The quality of the results are much more important than the volume of facelifts performed. Study your prospective surgeon's before and after facelift Photo Gallery to make sure that it is extensive and reflective of their practice. Make sure you like the results you are seeing in the photo gallery which should have a very natural appearance consistently.
There is no defined number. I have seen surgeons who get mediocre results but do a 100 or more in a year, and I have seen results of some surgeons who do 5 a year that look fabulous.