The Libertarian Party holds a virtual presidential convention on May 22. There are 1,046 delegates. This appears to be the largest number of delegates at the presidential convention of any party, other than the Democratic and Republican Parties, since 1948. In 1948 the Progressive Party national convention in Philadelphia had approximately 3,200 delegates.
In 1968, George Wallace’s party did not hold a presidential convention.
In 2000, the Reform Party national convention had 683 delegates. In 1996, the Reform Party held a national convention in Valley Forge, Pennsylvania, but the presidential nominee was chosen by postal ballot, not by that convention.
The Libertarian Party’s 2016 convention had 922 delegates, as determined by the largest vote cast for president at that convention, the second round.
The Green Party’s 2000 national convention had 317 delegates.