Arduino LEDs Novice

Hi all,
Right. I know next to nothing about electronics, but I have an Arduino and have been lumped into the hardware/software side of my final college project.

The project in essence is based around the gathering of people. The more people that gather in a certain area, the more LEDs that will light up. We'll either be using bi-tonal thresholding (camera picking up people against a light background) or simple switches (you stand on it, you flip the switch) to get the information from the oustdie world into the Arduino.

At the moment, I know how to rig up switches so an LED will light. And uh, that's about it.
Problem is, we're planning on having at least 50 or so LED's, and then won't be necessarily all in a line (we want them each in individual lantern-like objects). I've seen tutorials for LED matrixes? Would that be the way to go? To be honest, those tutorials make no sense to me, but if I knew I was on the right track I'd at least have something to work towards.

First question, if you could please help, how many LED's can be powered from the one pin? Is it possible to link up the LED's in maybe 10s, so that when a certain percentage of people are there, or if one switch is set off, 10 would light up, etc?

I have plenty other questions but first things first I suppose!

how many LED's can be powered from the one pin?

Each Arduino digital pin can supply 40mA, max. So you can run a maximum of four LEDs (at 10mA each) off one pin. However, you will probably be better off using some external chips to connect the LEDs, so that you can get more current drive, and so that you can mount the LEDs remotely. You may find it useful to search the forums for the 74HC595 chip.

Note although you can power several LEDs from one pin they all have to be on or off at the same time.

Sorry if this sounds obvious but I think from the words you used you could be missing that point.