You’re overflowing the maximum value.
An int’s max value is 2,147,483,647, and if you go over that, it loops back around to negative numbers.
PlayerPrefs doesn’t let you store long long numbers (max value 9,223,372,036,854,775,807)
SO, you’ll need to use something other than PlayerPrefs for saving that number, OR you could maybe store it in chunks of numbers. I’ve seen other clickers do something like every factor of 1000 or so, they just change a suffix.
So 1,000 would be stored as 1k
1,000,000,000 would be 1b
99,876,125,244 would be 99.87b, and the other numbers could be stored as 6,125,244 somewhere else off screen.