As an adult you can consume up to 7 bananas per day which would mean you'd consume the recommend level of potassium which is 3,500mg of potassium per day.
Of course you can eat more than 7 bananas per day if you want too and still be healthy and not get a potassium overdose but if you do consume more than 7 bananas per day you might get a stomach ache.
To get overdosed on potassium and potassium poisoning you'd have to eat at least 195 bananas in one day which is very unlikely because you'd likely get a stomach ache before you were able to consume that amount of bananas.
As long as you stay under 100 bananas per day you'll be safe.