Or can you tell me whats the general rule when using crosshatches, more denser materials have denser crosshatches etc?
Hatch Patterns depend on the part's material and the Standard you are using as reference. Each material has its own characteristic Hatch that allows the viewer to identify and differentiate materials in a Part or Assembly drawing. You should refer to technical drawings books such as “Engineering Drawing and Design” from Jensen, or directly refer to a Standard such as ASME Y14.3M-1994 (Multi and Sectional View Drawings).
Most CAD packages have automatic Hatch patterns that generate based on the material properties of the part.