Necessary knowledge in C #

On a scale of 1 to 10 how much knowledge about C# should I have to begin making games in monogame?

It depends.
MonoGame is an extension which uses all of the C# syntax, and therefore would atleast require basic knowledge of C# to implement. The greater you understand how C# works and features within, the more you can utilize MonoGame and its features.

td;dr - The more confident and familiar you are with C#, the more you can do with MonoGame.

You don’t need advanced knowledge of C# in particular, but a solid basic understanding of programming is required: difference between passing by value and by reference, declaring methods and types, using loops and conditions etc.