Do insurance companies take the number of witnesses into consideration?

Anything is possible. Since you don't yet know what the other party is claiming why are you concerned? If your witness is credible I doubt if a problem will arise, unless there are aspects of the accident that are seriously in question. Your insurers will fight your case and providing you have given them all the information that you can there is nothing more to do at this stage. Keep in close touch with your insurers so that you can refute untrue claims if necessary.