It depend upon the no. of cells present and the protein content in the cells. IF you don't know things, start with lower volume, measure the protein concentration and if that is too high then dilute the protein concentration with lysis buffer. Further  dilution depend upon the protein conentration observed when you added X volume of lysis buffer, and what is the final concentration you want to be in final samples.

Dear Heba,

count/estimate  your cells. Confluent, you usually have 1-3 mio cells in a 6WP. For good westerns, you take 100.000- 500.000 cells per lane (10-80 µg protein). However, all parameters pretty much depend on your cell line.

Best

christian