Hello,
I checked my sitemap.php but is incomplete compared to this forum's sitemap.
My sitemap is - https://raii.ro/forum/sitemap.php

I want to show as many as possible in the sitemap, as it is on the sitemap of this domain.
https://invisioncommunity.com/sitemap.php

The sitemap would show what you currently have content for or have enabled. 

Be sure you're using the default settings for the sitemap in ACP -> System -> Search Engine Optimization. That's all you need to do. As content builds out on your community, the software will automatically build out your sitemap.

https://i.imgur.com/9wYY6Ox.png

and isn't there a way to force the sitemap to add as many links as possible?

It would follow our recommendations. It adds all public content available which is of value to search engines.