You'll probably need at least ten to make it even recognizable as a plane.
I would probably use over 100 brushes to make a plane but that's because I'd have a lot of detail and I like planes... PLANES!!
You want to make the plane drivable?? Well you have to make it a func_vehicle then and I can't help you with that. Google for func_vehicle if you must do that.
You don't need
to use the slice tool. Try just the vertex manip tool (Shift+V).
The slice tool is the button with part of the block missing. Select a brush, click the slice tool. Then click 2 points on one of the 2D views to make a line. The line will slice the brush when you hit ENTER. You can move the line's endpoints around first if you want to.