No, you don’t have to be on camera at all if you choose not to.
Some people switch between having their cameras on and having their cameras off, depending on what they’re doing.
Please keep in mind that everyone is focusing on their own work and not on you. This is not about who looks the best or the worst or what your surroundings look like.
The major benefit of body-doubling is that you are in a head space where you perceive that other people who are out “there” are being productive, so it makes you want to be productive, too.
Having said all that… It is totally your choice if you want to keep your camera off or turn it on.
Some people rock side-to-side while working. If this is your situation, please turn off your camera while you’re on a focus session. That type of constant movement can be very distracting to other participants.