There is no concrete rule on calculating how many speakers you will need for a particular installation and how far apart you should place your speakers. The answer will depend on many factors such as your usage need/preference (specifically the preferred volume level), the type of amplifier/speaker you are using, and the surrounding environment.
In designing the optimal outdoor sound system, it's preferably to place more speakers and play them at moderate volume level than to place less speakers and blast them at high volume. Having more speakers at moderate volume level will prevent "hot zone" effect and provide more even sound coverage and better general sound for your outdoor space.
We simply suggest that you do not place speakers further than 40 feet apart. The ideal distance between speakers should typically be anywhere between 10-25 feet.
If you are interested to receive more specific recommendation for your project, please log a ticket with a description of your outdoor space with a rough estimate of the dimension. A sample photos or two would help us to give a better recommendation.