The four additional launch tubes in the VPM could carry a total of 28 additional Tomahawk cruise missiles (7 per tube), which would increase the total number of torpedo-sized weapons (such as Tomahawks) carried by the Virginia class design from about 37 to about 65—an increase of about 76%. The Navy wants to start building Virginia-class boats with the VPM in FY2019. Building Virginia-class boats with the VPM would compensate for a sharp loss in submarine force weapon-carrying capacity that will occur with the retirement in FY2026-FY2028 of the Navy’s four Ohio-class cruise missile/special operations forces support submarines (SSGNs).
Each SSGN is equipped with 24 large-diameter vertical launch tubes, of which 22 can be used to carry up to 7 Tomahawks each, for a maximum of 154 vertically launched Tomahawks per boat, or 616 vertically launched Tomahawks for the four boats. Twenty-two Virginia-class boats built with VPMs could carry 616 Tomahawks in their VPMs.
SOURCES - congressional research service