The subject says it all. :)
Rob.
If there is a limit, I doubt you will reach it. I have 6 on mine.
I have 11.
I think the only limitation you might come up against is if you use the Link Device feature, you are limited to 78 extensions. If I remember correctly, that number was recently increased from 20. So I can't imagine there would be any limitation south of 78 devices (includes softphones.)