I don’t know about Acts saying that there has to be a quorum of twleve, but the original Apostles did baptize many of the faithful, including leaders who continued the priesthood. I would hope that you would have a further interest in investigating this for yourself.
You mention the cessation of scripture, but it should be pointed out that Jesus Himself did not write anything down, nor did he ask or tell His Apostles to write anything down. Rather, He told the Apostles to go out to all nations, and baptize in the name of the Father, Son, and Holy Ghost, and to teach all that He had taught them. That is precisely what His Catholic Church has always done, since the beginning. That there is still One, Holy, Catholic and Apostolic Church which still holds to what He commanded and taught is miraculous. Objective truth does not change, so of course the teachings of the Catholic Church do not change, except in matters of discipline, and clarifications.
So even though Our Lord did not write anything down, nor instructed His Apostles to do so, His teachings were written down so as to have an account of what He instructed, to avert and prevent error. Our Catholic faith is based on Scripture, Tradition, and the Magisterium. It is only His Catholic Church that has the authority to teach and baptize, to interpret Sacred Scripture, and to continue to go out to all nations. Our Lord set up a heirarchical structure, beginning with St. Peter, the first Pope, because a governing heirarchy and structure was, and still is needed to properly carry out Our Lord’s mission here on earth - which is ultimately for the salvation of souls.
If any other Catholics here can further clarify this, or point out if I’ve gotten something wrong, I’d appreciate it.