If you're looking to have a full 360 video degrees video with unmodified GoPros, the good number is six.
You can order a rig directly from our shop to do so on the 360 camera section.
We recommend Freedom 360 broadcaster rig for Vahana VR.
If you use a modified rig with a solution such as the Izugar Z3X, then you can use only three modified GoPros, which means less cable and faster setup. For a situation where high resolution is not needed, it's a good trade-off.