How many participants can be visible at a time?
There is no limitation of how many video senders there will be in a conference. There is a hard limit of 75 people in a conference, though and we are working on increasing that.
And there is no limit on how many can be visible at once? Can I for instance see ten interlocutors at once?
On meet.jit.si yes, on your deployment you can set lastN value which will limit those.