Godot will support import in at least 5 types, Windows, Linux, MacOS, Android, Browsers (html5). What platform should be required for the game to be rated? and in general, will the number of people who have played the game be important? (of course, it is important, but I still wanted more information and specifics) 

Hey! Great question - I believe when you create a new 'project' for your game on itch (before you submit it to the jam) you can make it available for more than 1 platform. For previous jams we've hosted we have had entries that are both run in the browser and run locally. 

Given the desktop OS marketshare (Windows: ~85%, Mac: ~12%, Linux: ~2%), I'd say windows is probably the safest bet there - although that being said everyone will be able to play it if runs in the browser!

Hope this helps.

go for a combination of the browser and an offline downloadable version of windows and either mac or linux (if you use either of those OSs on your main computer).