Fathom’s run-time CPU load is displayed in the CPU bar at the upper right of the interface. If you do not have multi-core processing enabled in Reaper, and the Fathom CPU is in the orange or red, you will only be able to run one track of Fathom. If the CPU bar is near 50% then you will only be able to run two tracks of Fathom, and so on.
The following settings will enable multi-core processing in Reaper, which is necessary to run multiple tracks of Fathom. These settings here are different then what you will typically find in forums, but these are the settings which were proven to work in testing.