How many items can be stored in a ListProperty? Is there a limit?
Entities are limited to 1MB in size (when encoded as a Protocol Buffer), so this provides a practical limit on the size of a list. Further, if the list is indexed, you're limited to 5000 entries before you get an exception because your entity has too many index rows.
In my experience you will get MemoryError exceptions deserializing ListProperties before you hit a hard limit. That used to happen to me with 5,000 to 10,000 Key entities in the list.
Also, entities are limited to 5,000 indexed properties so 5,000 is a good maximum. (I feel like I used to have more sometimes but I don't remember exactly now.)
来源：https://stackoverflow.com/questions/1914740/whats-the-max-length-of-a-listproperty