TT #52 · Parshas Tazria · Aliyah 1 · Passuk 7
English: He shall offer it before GOD and make expiation on her behalf; she shall then be pure from her flow of blood. Such are the rituals concerning a woman who bears a child, male or female.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:7
Torah Temimah cites Kerisos 9b for the rule that one childbirth offering can cover multiple births, subject to the verse’s limitation.
E-Daf — Kerisos 9b · Shas.org · Sefaria — Kerisos 9b, source text and translation
English: “This is the law of the woman who bears a child” teaches that she brings one offering for many births; “this” limits that rule regarding births before and after completion.
Textual limitation
The general rule permits one offering to cover multiple births, but the word “this” limits combining events on opposite sides of the completion of the purification period.