I have contacted an expert on this topic and they replied with a few considerations.
Are these items part of an archive that has been processed at the folder level? Are the slides individually accessioned and considered the actual objects?
If they are in an archive they probably weren’t accessioned as individual objects, so you could just count the slides in each carousel. If they were accessioned as individual objects, then they should be inventoried individually.
Your decision also depends on the goal of your inventory. What kind of information are you gathering and which option will help you achieve that goal?
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