I tried the endpoint. 5 digits always gives you a valid user, 6 stops working after some point.

They probably meant 6 digit and it was a typo. The rest of the article references 6 digits. If it's just an integer (highly likely) it would go up to 10 digits or roughly 2 billion max users. My old coworkers and I used to joke that hitting INTEGER.MAX_VALUE for your customer ID is a good problem to have

Unless by “digit” they mean any alphanumeric.

You’ve got a lot of options if you go to a 5 characters with letters.

Which they’d kind of have to with 700k users if it’s 5 characters.

If it’s case insensitive, you’ve got 60,000,000+ combinations, and if case sensitive then 916,000,000+ combinations.