Let's consider Troi for a moment as an example of a Betazoid's telepathic range. Someone noted this above about a page or 2 back, but it's worth exploring.
The FX scenes really distort scale. Whenever Troi reads the emotions of a ship's captain whom Picard is sparring with on screen, that ship captain is tens of thousands of Km away. They really aren't within a few feet of each other.
Earth's diameter is 12.75 thousand km. Circumference is 40 thousand km. A typical class M planet.
So, if Troi can read the emotions of a specific individual that's tens of thousands of Km away, then she should be able to read everyone on the Enterprise from wherever she is on the ship simply by focusing or concentrating. Likewise, the same holds true if she's on a planet - anyone within range is open to being read.
Let's assume this is typical for a full Betazoid. At any given time there are anywhere from a few hundred to a few billion people within telepathic range. If, as Nyotarules keeps emphasizing, non-telepaths just broadcast their thoughts the way people have conversations in public, then that's upwards of billions of "voices" telepathically bombarding the senses of a Betazoid (or any telepath) all the time without letup.
With all that noise, maybe it takes extreme focus to pick out 1 specific individual. Does a telepath really need to work at not eavesdropping on the billions of stray thoughts bombing their senses or does it all become telepathic white noise?
That is, if there is a distance range limit. Perhaps it's line of sight and the target must be seen either in person or on screen in order to be read by a telepath. Can't see them? Can't focus on them.
Then again, Spock made a telepathic connection through a wall to an unseen target on the other side, Tam Elbrun made contact with Gomtuu across light years (didn't he?), Wyatt Miller did the same when he made telepathic contact across light years through Dreams. Spock even sensed the loss of the Intrepid from light years away.
So, who knows?