They've done it before.

"Article III of the United States Constitution does not specify the number of justices.... Congress added justices to correspond with the growing number of judicial circuits: seven in 1807, nine in 1837, and ten in 1863.
...
President Franklin D. Roosevelt attempted to expand the Court in 1937. His proposal envisioned appointment of one additional justice for each incumbent justice who reached the age of 70 years 6 months and refused retirement, up to a maximum bench of 15 justices. The proposal was ostensibly to ease the burden of the docket on elderly judges, but the actual purpose was widely understood as an effort to "pack" the Court with justices who would support Roosevelt's New Deal.[78] The plan, usually called the "court-packing plan", failed in Congress."