Is it permissible to leave the Tarawih prayer after the tenth rak'ah if it is more probable that it is eleven rak'ahs, or is it better to complete twenty-one rak'ahs with the congregation?

It is better to complete the prayer with the Imam until he departs, even if it exceeds eleven rak'ahs, due to the generality of the Prophet's (peace and blessings be upon him) saying: "Whoever stands with the Imam until he departs, Allah will record for him the Qiyam (standing in prayer) of a night." And because the night prayer is two rak'ahs by two rak'ahs, then if one fears the dawn, he should pray witr with one rak'ah. So, if the choice is between leaving the Imam due to the number [of rak'ahs] or staying with him if he adds more, then it is better to stay with him while advising him to adhere to the Sunnah.