First choice was to count albums and not objects - not 100% sure that was a good choice now, but e.g. selecting 10000 NAT files would be a rarity - so counting albums still seems ok. In the end I thought albums was best because it was something that the user could count easily and so they would get a consistent experience 10 albums ok, 11 albums too many.

Second choice was how many - I wanted a number (of albums) which was large enough to cover most useful analysis scenarios, but small enough that I could test it and small enough that performance was reasonable. On my dual-core laptop, 10 albums (or typically c. 200 tracks) seemed to be reasonable and retain sub-second responsiveness, but 20 or even 50 albums would probably still be OK (i.e. not lock up for more than a couple of seconds).