All Osmo games can be played collaboratively. So if several kids want to play the same game at the same time, one Osmo base is enough.
They can also take turns using one Osmo base with different devices, as long as it's the right type of base (iPad or Fire), and as long as their devices are compatible with Osmo.
However, if they want to play different games at the same time, you will need to have a separate base for each device.
We suggest purchasing one Starter Kit to try the games out, and later adding some games or bundles for different ages and skills, and more iPad or Fire bases if needed.