About btckeys.ru/en/
If you want to know how many Bitcoin private keys actually exist, I can recommend you watch this video:
Private key generation btckeys.ru includes all bitcoin private keys. These keys are not stored on the server as this would require an incredible amount of disk space. Instead, keys are created every time the page is loaded.
On each page, 64 private keys are generated, for example, on page 3 the numbers from 2*64 + 1 = 129 to 3*64 = 192 will be used. This simple formula is repeated for each page until the maximum value of 2^ is reached 256.
This means that all possible Bitcoin private keys have already been generated and are available on the website btckeys.ru. However, the likelihood of finding a key that matches an existing wallet is extremely low.
Therefore, the site is more of an interesting experiment than a real tool for finding Bitcoin private keys.