This sounds like it's related to SWS and should probably go to the
SWS issue tracker.
Anyway, you can try this:
Locate Reaper resource path (you can run the action 'Show REAPER resource path in explorer'), open S&M.ini which is found there in a text editor, locate the [TrackTemplates] section and set Max_slot to your desired value, save the file and (re-)start Reaper.