Hi Jurgen,
Why not just use default values and number them yourself to begin with. For new rows, you can have a rule on the repeating control that just sets your number column using the same formula. That way it only fires when a row is added and you get the first 31 (or whatever the default is) auto-numbered so you won't hit the browser issue.
I suspect that you are using a default value and some kind of insert logic to create 31 rows and that is what's causing the issue. By itself, the default value should not, but it's easy enough to just use default values - that's what they were intended for.
Good luck!