Number of justices has nothing to do with anything. FDR didn't seem to think so, after he threatened to pack the SCOTUS with additional Justices they became much more compliant.
Court decisions have limited the power of government on many occasions not least was the ruling that the Income tax was unconstitutional. Why would a bunch of lackeys have done such a thing?
Its an honor system. There is no inherent motivation for the three branches to limit the power of the government.
And why would they have ruled Congressional acts unconstitutional with some regularity starting with Marbury v. Madison?
That was a power grab by the SCOTUS with obvious conflicts of interests. Prior to Marbury every branch of the government was expected to evaluate the Constitutionality of its actions. After Marbury, the SCOTUS reserved that privilege solely for itself hence Bush's: Gee CFR looks unconstitutional but who am I to judge, where do I sign?
Today you can't even mention the 2nd Amendment to a jury in a lower court lest they begin to contemplate matters of Constitutionality.
Never the less Congress stopped FDR. And, as the Court history shows, there was no guarantee the new justices would have rubber stamped FDR's plans. Ask Jefferson and Madison about that.
The constitution provides the "inherent motivation" to limit government. Ruling the Income tax unconstititional shows that the Court can work to do just that.
Marbury was not the first case which went before the Court as a matter of constitutionality. Alexander Hamilton argued before it that the Carriage Tax was constitutional about ten yrs. before. Ironically, Marbury actually prevented the Congress from enlarging the court's jurisdiction.
Discussions at the Constitutional Convention indicate that judicial review was assumed by the Founders. They understood that it was useless to hope that the other branches would limit their activities merely because they were believed to be unconstitutional. Congress ALWAYS thinks its laws are constitutional as the Executive ALWAYS thinks its acts are. Only the Court has no conflict of interest in ruling on such matters.