It’s difficult to get a definitive number because it’s likely nobody has eaten a fatal dose. Estimates of the number to kill a person seem to range from 250 to many millions.
There was a rumor going around that eating 6 or 7 bananas in one sitting could kill you. The primary reasoning was that the potassium in a banana, consumed in a large enough quantity would result in a fatal overdose. Snopes.com has done a fact-check on this, and concluded that no, 6 or 7 bananas will not kill you.
The great Karl Pilkington gave his opinions on this subject on his show An Idiot Abroad.
The other concern, apparently, is that bananas naturally contain a small dose of radiation. Potassium decays and is very slightly radioactive.
“Other ‘radioactive’ potassium-rich foods include spinach, white beans, apricots, salmon, avocados, mushrooms, and yogurt.”Let me ask you this: ‘Are bananas radioactive and is it true that eating too many can kill you?’ | The Independent
By some estimates, you would need to eat roughly 10,000,000 bananas in one sitting to receive a fatal dose of radiation. Or if you wanted to poison yourself slowly, you could do it with a few hundred bananas per day (274 every day for 7 years).
So, if you were holding back on eating too many bananas for fear of receiving a fatal dose, you can relax. EAT THOSE BANANAS!