I don't know the exact number of cells I use at the time of lysate preparation. However, I use 80% confluent cells grown in T150 cell culture flask. For Lysis, I incubate the detached cells in lysis buffer (0.5 mL per flask ) for 40 mins on ice with intermittent vortexing. In this manner I was able to achieve 1mg/mL protein. Meaning, 0.5 mg protein per flask.