U.S. Strategic Nuclear Forces: Background, Developments, and Issues [Updated December 14, 2021] [open pdf - 2MB]
From the Summary: "Even though the United States has reduced the number of warheads deployed on its long-range missiles and bombers, consistent with the terms of the 2010 New START Treaty [Strategic Arms Reduction Treaty], it is also developing new delivery systems for deployment over the next 10-30 years. The 117th Congress will continue to review these programs, and the funding requested for them, during the annual authorization and appropriations process. During the Cold War, the U.S. nuclear arsenal contained many types of delivery vehicles for nuclear weapons. The longer-range systems, which included long-range missiles based on U.S. territory, long-range missiles based on submarines, and heavy bombers that could threaten Soviet targets from their bases in the United States, are known as strategic nuclear delivery vehicles. At the end of the Cold War, in 1991, the United States deployed more than 10,000 warheads on these delivery vehicles. With the implementation of New START completed in February 2018, the United States is limited to 1,550 accountable warheads on these delivery vehicles, a restriction that will remain in place through February 2026, while New START Treaty remains in force."
CRS Report for Congress, RL33640
Congressional Research Service: https://crsreports.congress.gov/