You cannot hack a Whatsapp account remotely just by knowing someones number.Whatsapp has a parent company named Facebook and it has real tight security. You need strong hacking skills,very powerful machine and tactics to break the shield of Whatsapp.
Your Whatsapp can be tracked if and only if your Government wants to track you and it makes a special request to Whatsapp company. It is really a long procedure and generally targeted to Government officials and celebrities. They don not track to find whether your girlfriend is cheating or not. They don not even track for fun.
People can read your message just by copying Whatsapp database file and encryption key from your phone and with help of some software, they can read it.