Is it ok to have 1 single storage account as checkpoint store referenced by multiple app services that use event hubs? Or is it better to have 1 storage account per app service, as it is recommended for azure functions here?
For performance reasons it is recommended to use seperate, dedicated storage account per event hub. As the docs say:
It's important to use separate storage accounts since Event Hubs triggered functions can potentially have a high volume of storage transactions due to checkpointing.
So it is the checkpointing that can cause high volume of transactions. That is why you should seperate the storage accounts used by the webapp/function/whatever from the ones used by each event hub.
There are virtually no extra costs attached to having multiple storage accounts if that is your reasoning for this question.