New states and boundary adjustment, etc
Section 8(1)(c) · 1999 Constitution (2004 edition)
What Constitution says
(c) the result of the referendum is then approved by a simple majority of all the States of the Federation supported by a simple majority of members of the Houses of Assembly; and
This is the 1999 Constitution as published in 2004. Later amendments aren't included yet, so a section may have changed since.
Original source
Official Gazette · CFRN 1999 official PDF · printed page 22–24. Original document
Release: draft-c8d50d87fae9. Source scope, review and corrections