LED's don't run on 5 volts.
The average red LED has a forward voltage of about 2.5 volts. This means that it doesn't really start to conduct until the voltage across it reaches about 2.5 volts, a bit like a zener diode. Once the voltage does reach 2.5 volts, the LED draw as much current as supply can handle, or as much current as it takes to destroy the LED, whichever comes first.
So, when you want to use an LED with a 5 volt supply, you need to do the following simple calculation:
First, you take the supply voltage (5 volts in your case)
Next you subtract the LED forward voltage drop(let's call it 2.8 volts)
This gives you a figure of 2.2 volts.
Now you have to work out what value of resistor will drop 2.2 volts at 25mA (if that's the current you want to flow through the LED).
Applying Ohm's law, R = V/I (where R = resistance, V = voltage and I = current) we get R = 2.2 / 0.025
Which means that R = 88 Ohms
Now let's suppose that you decide to wire two LED's in parallel, sharing a common current limiting resistor, commons sense tells you that you need to use a resistor of half the above value, i.e. 44 Ohms, in order to supply twice the current. However, all LED's are not created equal, and in the real world, one will have a slightly lower forward voltage drop than the other one, and will end up hogging most of the current. At some point it will probably burn out, at which time the remaining LED will take all the current (50mA) and it will soon burn out.
You buy get LED's which run from 5 volts (or 12 volts, in fact). They have built-in current limiting resistors, and are usually a bit more expensive.
So, as you've probably worked out by now, there is not much advantage to using your 5 volt 10 amp PSU, you could merely substitute 12 volts in the above calculation. The only difference is that the 500 current limiting resistors would be dissipating about four times as much power as heat. But then again, what is the regulation method used by your 5 volt PSU? If it's based on a linear regulator, it will be dissipating approximately the same amount of power as heat. In the long run, it would be way easier and cheaper to get a big bumper sticker printed saying "Hey everybody, look at me! I'm really cool!".