How many LEDs can I power using a single GPIO pin?

Hi all, so basic question for anyone that really understands electrical engineering. I'm still a newbie and getting conflicting information when I search for it so I want to ask here. Using a single 3.3v GPIO pin and the LEDs having a 3v forward voltage, and needing 20mA, how many can I connect in series?

What I want to do is create a couple of LED lamps (using multiple GPIO pins) I can install on my 3D printer to shine downwards to illuminate the bed while printing.