I used a fluidsynth-dssi in each track and hit the limit of 15 instances that worked as expected.
I could increase this limit by building fluidsynth-dssi from source with "#define FSD_CHANNEL_COUNT 32" instead of 16 in fluidsynth-dssi.h
I am afraid a standard build of fluidsynth-dssi will hit the 15 instruments limit, no matter how many instances you load in your session.
I used a fluidsynth-dssi in each track and hit the limit of 15 instances that worked as expected.
I could increase this limit by building fluidsynth-dssi from source with "#define FSD_CHANNEL_COUNT 32" instead of 16 in fluidsynth-dssi.h
I am afraid a standard build of fluidsynth-dssi will hit the 15 instruments limit, no matter how many instances you load in your session.