Yes, now I found the limit in the code anyway, I was blind. There is a limit.
It seems like it can be shared with 10 as read-only, and 10 read-write. This wasn't really the idea, and it should be checked client side first, which I have forgotten. And better feedback should of course be there. This is something I have missed to write down in the design doc, and therefore missed to implement.
Honestly there is no reason to have such low limit as 10. It was more that we wanted a limit, 10 was chosen because it also fits the simple UI. But it could just as well be 50, it wouldn't really affect the site.