One screen equals one active stream
A simultaneous screen is one active stream. Installing an app on multiple devices does not necessarily allow them to play at the same time.
You can usually set up your subscription on several devices, but only the number of screens in your plan can play at once.
What happens if you exceed the limit
The behaviour depends on the provider. Typically, the extra stream will not start, or an existing stream may stop. Check the plan details before buying.
How to choose
Count how many people are likely to watch different things at the same time — not how many devices you own.