How many rocks do I need?
That would mainly depend on the size of the area you wish to cover with sound. For a small space such as around a hot tub, you could get away with just one, although two would be required for stereo sound.
Larger open areas would require at least two, many times four, to achieve the dispersion level needed to cover that area in sound.
The greater majority of our customers buy two. Those with large pools, tennis courts etc. buy four. We won't try to sell you more than you need, and bear in mind that more can always be added later.
But that's not to stop you from using just one in a waterfall display, or just sitting next to your favorite chair out in the yard. They look good no matter where you place them.