It depends on your application and your camera. If you shoot yourself with a normal background without cropping, one is enough for now. Then you have a soft shadow on the face. With a second light with lower luminosity you could also illuminate the shadow (keyword: fill light), but you wouldn't need it at the beginning.

If, on the other hand, you want to use a green screen to cut out your face and place it in front of another shot, it depends on your camera how much light you need. With a simple webcam, you can't avoid lighting the greenscreen separately. The better the camera, the less light you need to crop. Then you could also use a front light to illuminate yourself and the green screen, but you will get green light back at you from behind. You can colour the green cast of your silhouette grey with OBS using "Spill", so that you can live with it for the time being. For professional cropping in front of the green screen, you need separate lighting, but you don't need that to start with. With my camcorder, which I use as a webcam via a cheap Capture Card USB stick, 2 keylights are enough for me and the greenscreen, while my Logitech C920 webcam can't produce a clean cropped image.

My advice: Start with just one key light!