Ah, okay. So 81 electors should have been appointed, then?
Two electors from Virginia, and two electors from Maryland did not vote. The Constitution calls for the electors to gather in their respective states to cast their electoral votes. Since the electors were chosen by popular election in Virginia and Maryland, I assume that the 4 above were unable to attend. Modern state laws provide for handling of absences and vacancies, typically by those present choosing replacements. I think this last happened in 2000, when a couple of electors were waylaid by weather conditions.
The New York legislature was to have appointed its 8 electors, but deadlocked and failed to choose any. North Carolina (7 electors) and Rhode Island (3 electors) had not yet ratified the Constitution.
So the 12 missing are NY(
, MD(2), and VA(2). All 69 electors who voted cast one vote for Washington. They split their second vote among 11 other persons. Adams (34 votes) was elected Vice President by virtue of his finishing second behind Washington.
The pre-12 Amendment Constitution provides that "The Person having the greatest Number of Votes shall be the President, if such Number be a Majority of the whole Number of Electors appointed, ..."
So Washington was elected by virtue receiving 69 votes from the 73 (or 81) electors appointed, with 69 of 73 (or 81) being a majority.