Where did you hear that? I hope your youtube video is not misinformed as you are, because sha256 has nothing to do with private keys. And if want to know how hard it is to "guess" or generate a key twice, considering your wallet is using standard implementation, you can right down 78 digits in decimal and see how hard it really could be.
Your number which you can pick, must not have first three digits greater than 115.  Because this is the limit, any number greater than this would be invalid: