You can do everything in 6 well plates. You will need replicates, always replicates in case something goes wrong in one of the wells. Duplicates or triplicates is standard. The controls can also go on the same plate if there is room. For each different day of drug treatment though you would need a control set for that day. So if you were doing 5 days you could set up 5 plates, one for each day and put both the experimental and controls in that plate and when the day has arrived you can just harvest the entire plate. Controls need to be replicates as well. 

You can use 6 well plates for your experiments. Its always better to have all your samples in triplicates and you can seed your cells in different 6 well plates depending upon the timeline of your experiment. For western blotting experiment you can seed up to 5 million cells each well followed by treatment for a days or two. Then you can harvest and go for total cell lysate preparation.

Depending on the cell line and its doubling time, you would need to standardize the number of cells per plate to start with. If this part has already been accomplished, then you are good to go. Typically 6 well plates work well. And like everyone else has mentioned, duplicates/triplicates will definitely help.Good luck!

But even if you didn't count your cells before seeding you can just determine protein concentration in each replicate and equalize all the amounts so that you run the same for all lanes. So long as you plates the same amount whether it was absolute cell number of volume just normalize the amount of protein for each well so you are loading the same amount   Your loading control should tell you as well if you did this well. 

As mentioned by others, 6 well plates would be a good option. In case you have difficulties, you may use T25 flasks, that have worked perfectly well for us.