This is a tough one to answer simply because every FlowRider in the world is different, but here are some things to think about.
Some of them are indoors, some are outdoors. Of course, if the wave is outside, more evaporation will occur due to the sunlight.
Some have a constant flow of advanced riders, some have mostly novice riders. The more advanced the rider, the more likely they are to perform tricks. When more tricks are performed, more spray is created. The more spray that is created, the more water loss over the edge of the ride.
Some FlowRider venues operate with 1 pump for some sessions and with 2 pumps for others. So, there will be a difference in spray & mist in the air with those difference operating modes.
All in all though, it’s not that much water. Keep thinking about it like a regular swimming pool; with a pool, you can expect after really hot days or after heavy use that more water will be necessary.
If we took misting, spray, splashing and water loss due to back washing (filter system) out of the equation then one may expect around 1,500 gal per week for a FlowRider Double. This would be Average evaporation based on average temperatures. However, there are so many factors here that affect water loss so it really is not worth trying to guess.
Sidenote: Your pool specialist will be installing an autofill system in the tank – just like with some swimming pools! If this portion of the filtration & tank system is in working order, then your FlowRider will be in optimum operating condition.