Matt,
Agreed with some points, but here are my reasons why I believe 16 is a magic number for BIAB.
It would probably make no sense if the aim is to have a plugin based BIAB only at some point, because most likely you would be able to open 2 (or more) instances of BIAB in DAW in the future to make up any number of tracks you desire... but I sincerely hope that while plugin is being aggressively developed, a standalone will remain in place... or at least would be launchable as a standalone with shell (future Nanohost or similar) Now before I say anything further, I did have a "wish" at one point to have tracks on demand... "+"

So here it goes:

1)16 tracks will easily fit in the screen and as with most (VST) sequencers/synth/samplers it would be familiar to most users.

2)Since BIAB is mainly acoustic/electric instrument program, usually 8 tracks are enough to "make up" the backing band, however my main reasons to have extra tracks for:

a)having a different variant or regeneration of RT/RD/Midi to compare side by side with ones that are already in place or auditioning various tracks with context of composition, without ridiculous workarounds or "losing" one in the process of opening/closing.
b)use the sections of any from above, by muting unwanted while still retaining whole tracks.

3)use extra tracks for filaments: Breaks, loops, minor segments etc.

4)just use extra RTs/RDs/Midi for "fuller" band

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I am not saying that I will be against 19 or 23 tracks, I would welcome that too smile
However, I do believe "16" will satisfy vast majority of users.