Read up on led matrix and/or charlieplexing.
You will have to consider the maximum current that a pin can sink/source, the maximum current that can flow through the microcontrollers port and the maximum power that can flow through the microcontroller's internal Vcc and GND.
Note that a LED does not have a common anode, it has an anode and a cathode; so you probably have either a RGB led or you have a display.
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