travel permit to Tibet.

We are Israeli citizens that work in Singapore and hold work permit in Singapore.
We would like to travel in China and Tibet this summer. to get a travel permit to Tibet how many of us have to be in a group? Can we get the travel permit to Tibet through the embassy of china or only through the travel agency? If we go to Tibet and return from there to other parts of China do we have to ask for Double entry visa or single one will let us to complete our journey?
Thank you for your help
Best wishes
Jenny