Back during the "slot match" days in Barry, Tulsa, and Vegas (all of which I attended), we peaked at around 40 wheelguns, then started a steady decline down to 17. It was never hard to get slots, and in fact I don't think anyone was ever turned away. 50 or 75 revolvers would have been no problem at all, if we had that many interested players.
Having Revo Nats in Barry as an adjunct to the SS Nats has been a mixed bag. It increased attendance due to the captive audience, but many feel that a one-day match doesn't feel like a real national event.