[houeto]

...Supreme Court cannot overturn any state or federal statute on constitutional grounds unless 7 of the 9 justices concur.

Who has the power to say how many 'justices' sit on the Supreme Court?

[Cboldt]

Congress. See FDR’s “Court packing” plan. It worked wonders, SCOTUS adopted a creative view of the constitution -on the spot-, even though the plan was never implemented.

[P-Marlowe]

Who has the power to say how many 'justices' sit on the Supreme Court?

Congress.