What should a follower do if he is certain of the number of rak'ahs of the prayer, but the Imam insists on adding more; should he continue with the Imam or leave him?

If the Imam makes a mistake and adds an extra rak'ah, and the ma'mum (follower) is aware of the addition, then it is not permissible for him to follow the Imam in it, otherwise his prayer will be invalidated. The ma'mum has the choice between separating from the Imam by intention and performing the tasleem, or waiting for the Imam to perform the tasleem and then performing it with him.