[Sesse__]

It doesn't really matter, since every position with 24 pieces can change into one with fewer. So you need the sum of all of those values.

For a more sophisticated estimation of the number of legal positions, taking into account _true_ legality (a legal position must be reachable by a sequence of legal moves from the starting position, however dumb), see https://github.com/tromp/ChessPositionRanking. You can probably filter their sample by number of pieces to gain pretty accurate estimations for number of 29-piece positions if you wish.

[zone411]

This matters in the context of your statement of "factor of 140 per extra piece," which doesn't hold when the number of pieces nears the maximum.