Do Jehovah Witnesses say only 144,000 will inherit heaven?

What if there were 144,100 people who were righteous and worthy? Would those 100 be cast out, condemned? This confuses me. If only that many can inherit heaven, why do Jehovah's Witnesses proselyte so they can add more to their thousands? I belive that all worthy people should inherit God's Kingdom, not just a set number of them.