Within platforms there is no limit to how many tablets you want to use it on! So for iOS users, you can use it on as many iPads as you own that use the same Apple ID that you purchased the app with OR are setup for family sharing.
For Android you may also use it on multiple android tablets if they are all using the same Google Play ID that you purchased the app with.
The apps cannot be used on multiple devices on different platforms though. Google Play purchases and iPad app purchases are two totally different stores/platforms that do not talk to each other so there is no way to allow the use of the app on both for the purchase of a single download. You would need to essentially purchase the app twice if you wanted to use it on both Android AND iOS platforms.