At times, it can be frustratingly obtuse as to why militaries decide to buy “x” amount of equipment. This page will collect historical assumptions used for past procurement decisions.

AEGIS Cruiser Procurement Assumptions (from)

Two ships for each Carrier Battle Group (12 CVBGs x 2 ships each = 24 ships)
One ship for each Surface Action Group (3 SAGs x 1 ship each = 3 ships)
Total Procurement: 24 ships for CVBGs + 3 ships for SAGs = 27 ships total.

Reference:
Naval Engineers' Journal, ©2009

U.S. SLBM Procurement and Deployment

The reason the U.S. Navy wanted forty-five SSBNs in the Sixties was because this procurement would provide a neat structure of five squadrons containing nine submarines each.

Reference:
From Polaris to Trident: The Development of US Fleet Ballistic Missile Technology by Graham Spinardi