Today I discovered something very curious!
I have a project with 18 players (26 instruments) and 8 flows.
I do not use any VSTi.
When I open the mixer and select MIDI the mixer shows a considerable amount of channels!
Several thousands!
I stopped counting as I noticed that I already counted 1254 and according to the bar at the top of the mixer it was about 1/5.
It seems a if Dorico has retained every track I ever created plus a lot more!
BTW the project is based on a template that I once created and tweaked and already used for several projects.
Well if you keep recycling the same project that’s blind to happen. Try going Play > Playback Template, setting it to Silence, applying, then going in again and setting it back to whatever it was on before (HALion Sonic SE?). It wouldn’t surprise me if your project sizes shrink.
Thanks pianoleo.
I just found this thread:
And I did what you suggest, the thousands midi channels are gone.
But why ist so?
Does it mean one has to care and “clean” or reset each project?
You surely know that there are good reasons to recycle the same project.
This is the only way to keep master pages.
In the meantime my master pages set is quite complete including all kind of pages for cover, content, empty pages and so on and it is quite a lot of work to redo this master pages set from scratch.
I’m not suggesting you shouldn’t recycle projects - I certainly do this here. When you remove an instrument from a project, the playback settings for that instrument aren’t removed. I’m guessing that this has a purpose: if you’ve removed an Oboe (for example) and later decide you want to add another Oboe, you can presumably hook it up to the Oboe playback settings you’d previously tweaked.
And “resetting” playback settings really is as simple as switching playback templates a couple of times, so I can’t see why that should be a hardship, particularly given the amount of time you save by recycling master pages etc.
I did not know this and I naively thought that when an instrument is deleted, well … it is deleted.
Hmm, I find that with your guessing Doricos team has indeed a good advocate 
I had to search to understand what “hardship” means, now I have learned a new expression 
And you are right, resetting the playback setting is not a hardship.
But one has to know that it must be done.
Well it’s genuinely a guess, and I’ve not actually tried it - it’s past my bedtime here.
The reason for this is that Dorico tries very hard not to change anything about the state of your plugins because we don’t want to overwrite changes that you have made yourself. For instance, if you change a sound in HALion Sonic SE (or other plugin), we don’t want to do anything automatically that might lose that data. So when Dorico allocates sounds for new instruments it will look for the first slot that it hasn’t yet used and loads sounds into that. So if you delete and add instruments then it will load more and more sounds. Leo’s suggestion is a good one, or alternatively, if you are using one project as a template then do this ‘reset’ on the template.
Thanks Paul for your explanations.
As soon as one knows about this then this not an issue.
Is this behaviour mentioned in the documentation?
If not, it may be usefull to mention it.
Applying the reset to the template is indeed the way to go.
Edit
@ PaulWalmsley
Concerning what you wrote about not changing anything about the state of the plugins I would like to add that most of the time I do not use any VSTi and no VSTi is loaded.