I've had several questions lately about powering LED spotlights, so I thought I'd see if I can't break it down a little better.
First off, there are 2 ways wire a group of LEDs - series and parallel.
Series: (or what you don't want to do)
In a series circuit, the negative lead of the first LED is connected to the positive lead of the next LED. and so on down the line. In this configuration only 1 resistor is used for the entire circuit. In a series configuration, the number of LEDs you can use is limited by the voltage of your power source. If you have 3 LEDs with 3.6 volt forward voltage wired in series, you couldn't use a 9 volt battery to run them because the total forward voltage is greater than the supply voltage (3.6 + 3.6 + 3.6 = 10.8 volts). I don't recommend this configuration for a few reasons, but mainly because if one point in the circuit fails you'd lose all your lights.
Parallel: (the easy way)
In a parallel circuit each LED has it's own resistor, and the positive leads of all the LEDs + resistors are tied together, and the negative leads are all tied together. When your LEDs are wired this way, the factor that limits the number of LEDs you can use is current (amps).
When you calculate the value for the resistor you need for your LED, one of the values you need for the calculation is the current rating of the LED. If we assume that the calculations are correct, then that current rating is the maximum current that will be flowing through the light. So all we need to do is make sure the total current of all your lights isn't more than your power supply is capable of providing.
So for example, lets say your LED has a current draw of 20 mA. 20 mA (mlliamp) is 20/1000th of an amp, so if you had a 1 watt wall wart, you could run 50 LEDs. I don't recommend loading a power supply to it's full capacity, so I wouldn't run more than about 40 LEDs (80% capacity).
It is possible to wire LEDs in a combination of series and parallel, but I'll go into that another time.