It's not entirely clear why, but the AF had about 5,300 of each in late 45. By 50 they were down to 577 F-47s and 907 F-51s and really only used the F-51s in Korea.
Following is the best data I could find, from
https://sobchak.wordpress.com/2012/...t-use-the-f-47-thunderbolt-in-the-korean-war/
"With the Air Force’s post-war fighter aircraft functioning almost exclusively as bomber escorts and air defenders, the Mustang was the fighter of choice during the transition to an all-jet force. During World War II, the Mustang was the premier long-range bomber escort. The Thunderbolt was limited in the long-range escort role by its notorious thirst for fuel, although Republic engineers did develop the F-47N, a long-range version of the Thunderbolt designed to escort B-29s in the Pacific. The N-model could fly 800 miles on internal fuel and as much as 2000 miles with external tanks, but it achieved this with a high fuel bill. The F-47N was similar to earlier models of the Jug in fuel consumption, burning 100 gallons an hour when cruising and as much as 300 gallons per hour at full power. The Mustang burned 120 gallons per hour at full power and as little as 64 gallons per hour at lower settings. (21)
As for the air defense role, Thunderbolt chronicler Warren Bodie acknowledges that the Jug “never was a good interceptor.” (22) The F-47 could not boast a great rate of climb, though with wide paddle blades and engine power boosted with water injection, late-model F-47Ds could reach 20,000 feet in nine minutes. The F-47N took 14.2 minutes to reach 25,000 feet, while the F-51D climbed to 30,000 feet in 13 minutes. The F-47D had a top speed of 428 miles per hour at 30,000 feet compared to the F-51D’s 437 miles per hour at 25,000 feet. The F-47N was able to achieve an impressive 467 miles per hour, but the F-51H was faster still, with a top speed of 487 miles per hour. Thunderbolt pilots in World War II were able to defeat their opponents through teamwork and careful exploitation of the Jug’s strengths–especially its diving speed, zoom climbing ability, and heavy firepower. Maneuverability was less critical against lumbering bombers but the F-47 could not match the F-51’s all-around ability in air-to-air engagements against enemy fighters. An exceptional Thunderbolt pilot like Robert S. Johnson might claim he could beat a Mustang “anytime I wanted to, and I did, many times,” (23) but Jug pilots often lost to the more agile Mustangs in mock dogfights.
Years of lean budgets and the neglect of tactical air power meant that by 1950 there were simply not enough Thunderbolts and associated spare parts left to support long-term combat operations. During World War II, 15,683 Thunderbolts were produced–more than any other American fighter. Of this total, an estimated one third were destroyed in combat, a third were scrapped after the war, and the remaining third went into storage, served with the Air National Guard, or were sold to foreign governments. Late-model F-47Ds and F-47Ns remained in service with a few active-duty Air Force units until the late 1940s, and the Air National Guard did not retire its last Thunderbolts until 1955. When the Korean War began, there were 1,167 F-47s on hand, but most of these were in storage–only 265 Thunderbolts were active in ANG units and they were all considered second-line aircraft. (24) Additionally, the rapid demobilization after World War II affected the supply system and the availability of spares for the Thunderbolts throughout the post-war years. For instance, the 23rd Fighter Group stationed on Guam in 1947 had pilots who had not accumulated the required night time flying hours because their Jugs lacked functioning flight instruments. The group’s historian noted “the installation of these instruments is contemplated in the near future, depending of course, upon Tech Supply.” (25) Historian Kenneth P. Werrell was told the F-47 was not used in Korea primarily because of the lack of spare parts. (26)"
There was one fighter that USAF quckly discarded after the ww2, the P-38.