English: “According to what each person eats you shall make your count” — this teaches that the Pesach is slaughtered only according to those who will eat of it.
On the daf / in context: See the cited source for the full section.
Type of Learning
Derash on the verse (Pesachim) — not a formal named middah of the 13.
What Is Learned
The Pesach is slaughtered only for those able to eat from it.