The Prophet (may Allah's peace and blessings be upon him) used to offer thirteen Rak‘ah in…

‘Ā'ishah (may Allah be pleased with her) reports that the Prophet (may Allah's peace and blessings be upon him) used to offer thirteen Rak‘ahs in the night prayer including Witr on a regular basis, regardless of whether it was during Ramadan or not. Likewise, he adhered to offering two voluntary Rak‘ahs of the Fajr prayer. It means that the Prophet (may Allah's peace and blessings be upon him) used to do this most of the time. This is because the Prophet (may Allah's peace and blessings be upon him) was reported to devote himself more in worshiping Allah in the last ten nights of Ramadan than he strives in any other time in the year.

It implies that the Rak‘ahs are increased in length, not in number. In fact, the Prophet (may Allah's peace and blessings be upon him) used to offer thirteen or eleven Rak‘āhs on some occasions and on other occasions he used to pray less than that.