the parameter is not required, if the number u entered isn't a valid ent (or not there), it just uses the ent u r looking at. If ur not looking at an ent, it will tell u a whole lot of nothing.
ents numbers are basically greater than 1 (0 = worldspawn?). I believe there is a maximum of just over 1000 ents. 1024 makes sense, but i've also heard 1065. There won't usually be more than 300 ents on a map (from my experience).