If number of items is too high, depending of the render engine, it may take too much time for rendering or generate some error.
Anyway the default limits (1 Mill items and 20 mill density) are high, and should let you to get a very dense distribution.
I suggest you you take a look to this FAQ entry
. It includes a technical description about the scattering system, and how to optimize it. The Rendering guide
includes some tips as well.
I hope that helps.