Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.

God waited patiently in the days of Noah, during the building of the ark, in which a few, that is, eight persons, were saved through water. And baptism, which this prefigured, now saves you . . . through the resurrection of Jesus Christ,
also We know, as a fact, that from the earliest times, baptisteries and fonts were octagonal. We know also that the reason assigned, if not by S. Ambrose himself at least by one of his contemporaries, for this form was, that the number eight was symbolical of Regeneration. For as the old Creation was complete in seven days, so the number next ensuing may well be significative of the new.
Rationale Divinorum Officiorum, pg.82, 1893 translation