How many people can I add to a conference call at the same time ?

That is a limit set by your cellular carrier for regular phone calls. It is generally a pretty small number. For FaceTime calls if everyone has an iPhone the limit is yourself plus 31.

For larger conference calls use a conferencing service (do a web search for conference calling) or Zoom. While Zoom is usually considered to be for video conferencing, it can also be used for conference calls. And it is free for calls lasting less than 40 minutes with fewer than 100 users. If everyone has a smartphone you can also use many other audio/video apps; Signal, Telegram, Skype, Google Meet, WhatsApp.