I have an Entity that represents a Payment Method. I want to have an entity group for all the payment attempts performed with that payment method.
The 1 write-per-second limitation is fine and actually good for my use case, as there is no good reason to charge a specific credit card more frequently than that, but I could not find any specifications on the max size of an entity group.
My concern is would a very active corporate account hit any limitations in terms of number of records within an entity group (when they perform their 1 millionth transaction with us)?
No, there isn't a limit for the entity group size, all datastore-related limits are documented at Limits.
But be aware that the entity group size matters when it comes to data contention, see Keep entity groups small. Please note that contention is not only happening when writing entities, but also when reading them inside transaction (see Contention problems in Google App Engine) or, occasionally, maybe even outside transactions (see TransactionFailedError on GAE when no transaction).
IMHO your usage case is not worth the risk of dealing with these issues (fairly difficult to debug and address), I wouldn't use a single entity group in this case.
来源：https://stackoverflow.com/questions/46411694/is-there-an-entity-group-max-size