The only thing our ordinance states is that private swimming pool clubs require one parking space for each two member family or individual, plus the spaces required for each accessory uses, such as a restaurant or bar.
Having said that, I like Mike D.'s suggestion about one for every fouth person allowed by fire marshall; however, I tend to be somewhat conservative on parking.
I also like Mike S' suggestion about reserving land for additional parking. We have this in our zoning to allow that:
"Where, at the outset of development, a parking demand less than that required for the use by this Zoning Code, can be demonstrated, the full parking area shall be reserved, but the unused area may be placed in landscaping until such time as it may be needed."