Those are fair questions. Surprized that support did not have a clear answer.

If they don't know, who does... but here is my input based on having all that are currently available.

Right now there are likely as many as 120 - 180 Playable Realtrack Sounds (PRS). Here is a picture of just those set up for Acostic Guitars. These have been made avialable over the past two year releases (2021 and 2022). I would have thought that these are now part of the "core" BIAB sounds which maybe enhanced with each new release. So a core purchase should get you all, IMO. Otherwise, how would they ever sell this feature if you could not get the sounds to use it? The current avialabilty may be sufficient to handly the need, after all we got by with what we had in 2021 before the additions in 2022.

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BIAB – 2025, Reaper (current), i7-12700F Processor, 32GB DDR4-3200MHz RAM, 1TB WD Black NVMe SSD, 2TB WDC Blue SSD, 1TB WD Blue, 2 TB SK NVMe, 6 TB External, Motu Audio Express 6x6