How many rerailers do you need if you are using a two train system? Two?
Rerailers are to help you put trains on the track. Since you can do this without the rerailer there is no minimum needed. Put on as many as you find convienent to have. Modular layouts sometimes use these near where the modules meet so if any derailments occur going over the joint they can hopefully be corrected. Others use them on hidden staging tracks to correct derailments.
Enjoy
Paul