how many lockons to use on Realtrax MTH

If...

You are running one transformer per loop, one lock on should suffice.  Also, making the assumption of one engine per loop.  While running a train, if it should slow down at some point on the loop you could install another lock on at that point and add a second run of wire from your transformer. 

This is all based on presumption that you are operating conventionally.  If you have DCS, you will need to create isolated blocks when running with more than one lock on.