Number of persons in a call

I have set up a group using free Microsoft Teams. We had our first group call/meeting last night however we could only see 4 of our members on the screen at any one time. The other members appeared just as a circle. When a non visible member spoke, then vision of them  appeared and someone else disappeared. It seemed as though the maximum visible at one time was 4 people.

How can we get all members on screen simultaneously?

Thanks

Judith