Read up on summaries. You can create a summary calc which counts any non-empty field. It counts based upon found set and must be sorted
Anoher option is to count the related Participants using a calculation field in the Choirs table - assuming you do have such table.
if you are running version 12, you could have a field that builds a report with the new and wonderful 'ExecuteSQL' function.
ExecuteSQL ( "
count ( member.id )
member.type = chior AND
member.active = 1
; "," ; "¶" )
This is build as if the table was called "member', and it had the three fields id, type, and active.