From the Summary: "On March 11, 2021, the U.S. Air Force took delivery of its first F-15EX Eagle II fighter. The Eagle II program is intended to deliver 144 aircraft to replace aging F-15Cs, most of which are in the Air National Guard. The Trump Administration's FY2020 budget proposal included a request for $1.1 billion to buy 8 F-15EX aircraft, the first procurement toward a planned initial buy of 144. This proposal represented a change from previous Air Force plans to procure only stealthy 'fifth-generation' fighter aircraft. The subsequentFY2021 defense budget proposal requested about $1.27billion in procurement funding for 12Eagle IIs and $133.5 million in advance procurement for future aircraft. The proposed budget also requested about $159.8million for F15EX research and development."
CRS Report for Congress, R46801
Congressional Research Service: https://crsreports.congress.gov/