How Do I Fix My Grass?

You've sort of answered your own question in your description: "My map is 20,000x 20,000 and I cant make a dense grass. On this 50x50 map I tested my method and it worked just fine on it". The solution if you want dense grass is to have many smaller terrains rather than one huge one.

Unity's grass system is somewhat arcane and esoteric, but it can be quite powerful when you understand it. Grass on a terrain is generated in "patches", which are stored as a 2d array of values (from 0-17, IIRC, from "least dense" to "most dense"). The size of that array is determined by the "detail resolution" setting, but when applied to a huge terrain size such as 20,000x20,000, the individual patches must be stretched to cover a large area so are individually very large. Because the amount of grass meshes in any given patch is limited (by the "detail density" and "detail resolution per patch" setting), this means that it's impossible to generate dense grass when you have large patches.

To generate smaller patches, use a smaller terrain size, and set your detail density and detail resolution patch high.