According to Skype, up to 25 people can use Skype at the same time when doing a group call, but there are limitations! If you are sharing your desktop, Skype say that only 10 people can use Skype at the same time. There may be other limitations that I’m not aware of, so I suggest that if you plan to use Skype for business purposes, that you find out what feature(s) you want to use, then set about seeing what limitations there are for that feature.
If you (or I) find that the number of people that can use Skype at the same time changes, please update this thread to help the next people reading this. Additionally, if you discover other Skype features that have limitations on the number of people using it at the same time, please share those below too. 🙂