Thanks, I now see that I had seen that.
But it seems it didn't stick in my brain.
I'll repeat what it says there, so it may be easier to find for others too:
$Count(<album>) counts the number of files in the same album
$Count(<album>, <album artist>) counts the number of albums for an album artist
$Sum(<play count>, <album>) sums the total play count for all tracks in an album
$Sum(<play count>, <album artist>) sums the total play count for all tracks by an album artist
edit:
And what about $Min and $Max?