In the .ReaperOSC file (i.e. OSC Control Surface configuration) you're using (*not* being Default.ReaperOSC), you can adjust the number of simultaneous parameters per effect slot, by changing the value in this line:
Code:
DEVICE_FX_PARAM_COUNT 16
to e.g.:
Code:
DEVICE_FX_PARAM_COUNT 24
And you can also dynamically adjust this from the control surface side, using the DEVICE_FX_PARAM_COUNT action description; e.g. using an OSC message like this:
Code:
/device/fxparam/count 64