If the plugin can accept multiple track inputs and has multiple outputs to feed to audio/instrument tracks then yeah, you just do the routing and a single synth would work.
You need to read the manual for the synth to find out what it's internal routing capabilities are. I doubt you are going to get 20 individual voices this way but you may be able to get away with only two or three instances of the synth IF it can handle "multitimbral" type input/output.
If it can only handle input from one source and output one signal then yeah... you have to insert a synth for each part.
In that case though you can use the "Freeze" option to freeze the synth instances (in the synth rack) to remove the synth from system resources. This will temporarily convert the track to audio and keep that instance from hogging system resources. You would have to unfreeze it to make any changes to your MIDI but (if you do it in the synth rack) you can still add/adjust effects.
So...
Read the manual for your synth to find out how to/if it can handle multiple inputs/outputs.
Read up of the Synth Rack "Freeze" function (not the "track freeze" function which will freeze your track effects).
Cheers.