Summary
There is no external specification about the max digits of a run id as of writing this article but it will be 19 because a run id is implemented by the Java Long type.
Question
What is the max digits of a run id in the Central UI and an execution id used in REST APIs?
Answer
In OO 10.x, there is no external specification about the max digits of a run id in the Central UI and an execution id used in REST APIs as of writing this article but technically it is 19 because an id is implemented by the Java Long type.