It is meaning number 4 in https://www.lexico.com/definition/body "A large amount or collection of something. ‘a rich body of Canadian folklore’ ‘large bodies of seawater’"

4 A large amount or collection of something. ‘a rich body of Canadian folklore’ ‘large bodies of seawater’ this definition from your link is what you are looking for. The available amount [body] of facts

There is a list of definitions for body. https://www.lexico.com/definition/body Which one do you think it is for 'body of fact'?