how many LED’s can one pin run if wired in parallel? how does one figure this?

how would it be achievable to run a large matrix of RGB led’s requiring PWM?


how many LED's can one pin run if wired in parallel?

40mA worth. Approximatey (that's SUPPOSED to be an "absolute max" current out of any pin.)

Less than one, if you don't have a current limiting resistor. If you DO have a single current limiting resistor, the resistor limits the current and you can attach as many LEDs as you want. They'll just keep getting dimmer and dimmer and dimmer as they share the current that you've allocated. How long before they get unacceptably dim will depend on the LED, and what you want them to look like.
If you have one resistor per LED, and multiples of THAT in parallel, the currents will add, and you are back to the 40mA limit. Now your brightness per LED will stay constant as you add them, though...

This topic (connecting LEDs to microcontrollers in general or arduino specifically) gets a LOT of discussion. Do a little research, please...