The following is the March 24, 2021 Congressional Budget Office report, The Capacity of the Navy’s Shipyards to Maintain Its Submarines.
From the report
The U.S. Navy’s 70 submarines represent nearly one-quarter of its total fleet. Like all naval ships, those submarines require several periods of intensive maintenance during their service life. Navy policy dictates that most maintenance on nuclear-powered ships (all U.S. submarines are nuclear-powered) be performed by the four shipyards it owns...
https://news.usni.org/2021/03/26/cbo-report-on-nuclear-submarine-maintenance