The best amount of drivers for a team is 2. One to drive the wheels, and another to drive the attachments.

Have your drivers practice anywhere, but driving on carpet is preferred.

The best practice is from actual driving.

Drivers need to be able to listen to their team mates and the coach. They also need to spend lots of time practicing.

Last thing, anyone can be drivers, but that doesn't mean everyone should. Make sure to select 2 drivers for the competition, and only allow them to drive. It makes them better at driving.