Peter asked Jesus this question and suggested perhaps seven times. As people do not usually forgive even once, seven times must have seemed very generous to Peter. Jesus told him seventy times seven. He did not mean multiply seven by seventy. He meant that there is no number and you have to forgive from the heart.
He gave the disciples a warning by telling them the parable in this passage. The servant owed a large debt to the king which he could not pay. When the king ordered it to be paid the servant begged for mercy and received more than he asked for. He asked for time to pay it, which he would have known was impossible as he could not pay such a debt in his lifetime. The king had compassion on him and canceled the debt.
One would think that having been shown such mercy, he, in turn, would also show mercy to others, but he did not. He found another servant who was in debt to him. He cornered him and told him to pay what he owed. Just as he had entreated the king, so the fellow servant entreated him and asked to be patient. He refused and had the man put in jail. The point is that if we are Christians, we have received mercy from God and we, in turn, have to show that same mercy to others or God will call us to account.
When Peter asked if seven times would do, he was really demonstrating that if you are counting, you are not really forgiving at all. Your heart is still not right and that is something we have to work on. Do we hold grudges, even if we say nothing, do we in our hearts hold grudges against other people? Remember, we are just like them. We are undeserving sinners, but God has been merciful to us and he expects us to be like him