guessing random private key From what I understand (which is not much), private key is like id/password for your wallet. So lets say, I have random number generator and I generate random number, is it possible that generated number could be key to someone elses wallet?
Yes, it is possibly, but not probable, or in other terms, extremely rare. There'll be maximum 2^160 addresses, which is much more than the number of addresses that are used. See this.