According to DoD, that’s the number of warheads in the US nuclear weapons stockpile.

The stockpile, the fact sheet says, includes

both active and inactive warheads. Active warheads include strategic and nonstrategic weapons maintained in an operational, ready-for-use configuration, warheads that must be ready for
possible deployment within a short timeframe, and logistics spares. They have tritium bottles and other Limited Life Components installed. Inactive warheads are maintained at a depot in a non-operational status, and have their tritium bottles removed.