Shared line maximum?

[glasses685]

I'm just curious if there's any specific limit or threshold for shared lines where it becomes a problem? Asking since I have approx. 15 phones sharing a line and I've had some sporadic reports that, occasionally, inbound calls don't ring to every phone. 

The phones themselves are mostly Polycom VVX 450s and they're all on the same network. I seem to recall being told in the past that shared lines become unreliable past a certain number of users, but I can't recall the number.