Unlike most plants which are decently full at 100, moss does take a lot of particles. But it should be thick at 1000.
If this doesn’t match what you see, you may have not turned off the Child Density Optimization. For moss, this can make it look a little thinner, it’s in the emission subpanel of the growth panel
Here’s what 1000 density moss looks like with the optimization
And i guess the last thing you may want to check is that you’re display is set to 100%, otherwise you won’t see all the particles.
Hope this helps.