Step 1: Identify the constitutional provision.
Article 124(1) fixes the composition of the Supreme Court, stating it shall consist of the Chief Justice and such other judges as may be prescribed.
Step 2: Recall who does the prescribing.
The words "as Parliament may by law prescribe" tell us that changing the number of judges needs an ordinary law passed by Parliament, not an executive order.
Step 3: Eliminate the other bodies.
Neither the President, the Central Government by notification, nor the Supreme Court itself has this power on its own.
\[ \boxed{\text{The Parliament by law}} \]