LEDs in Series

How many LEDs in series can be connected to a single arduino port. I don't need to do anything special, just power a lot of LEDs in series or parallel (about 20 - 30). They are going to be connected to a port just to turn them off or on. How much can the arduino handle? thanks!

See:

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2 tops if they are red.

You can wire a bunch in series/parallel and turn of/off with a single transistor an Arduino IO pin can control easily tho.

AOI514 from digikey.com will work well.