People can guess private keys? Seriously? No one is capable of doing that genius. Many have tried to crack these keys in different ways(Brute force etc), but failed which is why a large portion of BTC is lost basically.
Supercomputers could possibly crack them in the future, but it would still be really difficult.
Of course, it is not now. It is a potential problem. When the number of Bitcoin accounts is large enough. It will be easier and easier.
What the correlation between the number of bitcoin account and its possibility to guess the private key? No matter how much the number of bitcoin account is, the possibility to guess the private key remain the same (wont be easier) imho.