Associate Justices are appointed by the President, with the advice and consent (approval) of the Senate, the same as the Chief Justice. The President who appointed the most justices was George Washington, with ten (actually eleven, but one declined his appointment).
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George Washington had the opportunity to appoint the most Justices of the Supreme Court. He appointed 11, out of the 14 he nominated.
The number of Supreme Court Justices a President appoints equals the number who resign or die during his/her presidency (unless his/her presidency ends before he/she has the opportunity to appoint a replacement).
Franklin D. Roosevelt was the president who appointed the second most Supreme Court Justices. He appointed eight justices during his presidency.
Washington had to appoint the entire court of seven in his first term . Taft appointed six justices in his one term.
George Washington is the president that appointed the most Supreme Court Justices. There have been only seven US Presidents that served only one term.
1789, when the first Supreme Court (of six members) was appointed by Washington.
County judges don't appoint Supreme Court justices. At the federal level, they are nominated by the President and confirmed by a simple majority vote of the Senate.
Judges serving on the US Court of Appeals Circuit Courts are appointed by the President, then approved by a simple majority (51%) vote of the Senate. Most federal judges are appointed to their positions, but a few are hired directly. Federal judges are never elected.
Since the US District Courts are the most numerous of the federal courts, it only stands to reason that the majority of appointed judges are assigned to this level of the court.
George W. Bush, the 43rd president, appointed the most convicted criminals to his administration.