Yes, an invigilator (proctor) is required. There is no set ratio, but usually an exam cohort could be around 20-30 candidates. So the ratio could be between 1:20 to 1:30.
As above, we usually don't put an upper limit on the maximum number of candidates per exam session.
But usually around 20-30 candidates depending on your venue.
(There are venues where they can accommodate 70-100 candidates at a time with just one proctor, and that's fine with us.)
In terms of number of video cameras, we frequently recommend at least two cameras; one on each side of the room. This allows you to cover the maximum view of the candidates. If you would like to put more candidates at a session, say 50-100, then you may consider putting in a third or fourth camera to cover everything. The main goal is to be able to see that candidates are not bringing in any books or using any cell phones and such.
We pay on a monthly basis (this will be detailed in the Agreement that we will both sign).
Yes, we have a standard testing centre agreement that we can both sign.
Log into IITTI under 'Student Area'. Then click on 'Exam area'. You will see 'Test computer'. Follow the instruction on screen (basically, if you can see the picture of a mountain, you are ok.)