There's no formal limit, but practically I wouldn't expect more than 5000 - 10000 clients. Generally I'd guess the average is south of 1000.

The Care agent said they'll try the other number the next time they call. I've been asking around - the only cases we are aware of where this happens is due to antivirus or backup software.  Hopefully he can work with you to identify if it's something else, and we can hopefully do something about it going forward.