The density limit is used to avoid creating too many items, and the value is set by the Distribution Map->Max Density (in millions of items).
The default value is pretty high, so if you get the warning but the density at viewport is normal, it may be due to:
1) The items are not visible at viewport because the Display->Max items limit. You can increase it, or render the scene to check the real density.
2) The scattering area is too large (although it should be really huge to reach the density limit)
3) Most of the items are being discarded because the collisions checking (if it is enabled).
In any case, i suggest to use the General->Forest Stats tool. It shows some statatistic about the number total of items at viewport, estimated for rendering or hidden because collisions.