The statement that the linear combination of atomic orbitals generates an equal number of molecular orbitals is a fundamental mathematical and quantum mechanical tenet of molecular orbital theory, requiring no citation.
The claim is a foundational rule of quantum chemistry (specifically LCAO-MO theory, where $N$ atomic orbitals combine to form $N$ molecular orbitals). As a basic definition and textbook principle of chemistry, it is common knowledge and does not require empirical citation.