Is it ever necessary to have more than one back flow valve?
A single backflow valve placed in your outflow line would be all that is needed to prevent sewage from the city main backing up into your house. It sounds like this is all that would be necessary to satisfy the insurance company, but you'd have to ask them.

Multiple backflow valves would be designed to prevent a blockage in one of your internal drains from causing a drain overflow in the basement. Ask the insurance company if this is what they require.

EDIT

Back-flow valves are not required in my neighborhood of slab-on-grade houses in Dallas, but I have installed a spring loaded overflow in my cleanout in the yard. This protects me from a backup in the sewer under the street in front of our house, but this simple device ($4.00) of course wouldn't work to protect a basement.

The streets run east-west and the terrain slopes down from east to west. The neighborhood is bounded on the west by a street running along a park around a south flowing creek. Two houses on two different streets north of us have filled with sewage inches deep due to a blockage in the city lateral where it enters the city main under the creek bottom. This was decades apart in time. The sewage from uphill filled up the main under the street and then out the shower and toilets of the lower of the two houses at the west corners.

I knew the people in both houses and it was a horrible mess.