Is there a documented limit somewhere that mentions a maximum amount of account-ids for such requests?
A quick google search for maximum URL length indicates that it is suggested to not exceed a URL size of 2048 characters or a query length of 1024 characters. With ‘&accountId=x’ being over roughly 50 characters, that is being done quite easily.
For what it’s worth, we are using the /rest/api/user/email/bulk endpoint in production and after some testing have settled on a limit of 75 account IDs for that endpoint. We use this hundreds of time per day and it hasn’t failed a single time so far, so I think you can consider 75 to be somewhat safe. Realistically, you could probably even go higher, but 75 is enough for us so we never tried.
@sven.schatter FYI, we are seeing ‘HTTP 400 Request header is too large’ errors as of today with 75 account-ids in the URL.
The breaking point seems to be around 40 account-ids for us. I have set the limit to 35 to be save with longer base-urls and additional query parameters.