In the Casino Match Cover Club Listings I counted 23 match covers and another 16 variations of Silver Slippers making a total of 39.

Here are some Season's Greetings covers from their newsletter:

This is match cover number NV 604:

Here's a description of a variation of NV 604 (it's a slipper facing "left"):

I've never seen one but it's out there.

The only one I have facing left in red is this one. The Casino Match Cover Club doesn't list it so you can add another one to the total (40).