Hi, I'm pluma from ArangoDB.
Yes, you can have any number of digits in your keys. Actually keys don't even have to be numeric. Keys are strings, not numbers. For the exact list of characters that can be in a key see the documentation: docs.arangodb.com/3.0/Manual/DataModeling/NamingC…
To summarize: any alphanumeric string between 1 and 254 characters long is fine.