One (1) is recommended. The Sidekick Board is designed to attach to the right side of the rear tire of your G5 Stroller. The Sidekick board also comes with a grab handle so that your toddler can use it to hold on too.
This said, technically you can fit up to two (2) Sidekick boards on the Orbit Baby G5 Stroller, one on each rear wheel. However the “grab handle” only will attach onto the right side of the Orbit Baby Stroller frame and won't fit the left side of the handle frame.
Orby Baby has US Patent 8714582 on our Sidekick ride along board. When attached to your rear wheel, it resembles a skateboard for your kids enjoyment, and is able to attach to one side of your stroller, not in the middle or using both wheels, and it stays away from you accidentally kicking it with your feet while strolling. Because of this design and patent, you are able to attach a Sidekick board on both sides of the stroller