I've had burping (40 - 50 times a day) for about 6 months now. I'm concerned that it is an early sign of stomach cancer.

Reflux. This is likely a sign of some sort of indigestion or reflux. You can try anti-acid such as Pepcid (famotidine) to see if symptoms get better. You should also consult with your doctor as you may need upper endoscopy for direct visualization of your stomach.