How many people witnessed Uruguay defeat Argentina in the first FIFA World Cup final at the Estadio Centenario in Montevideo?

68,346 spectators

https://fifawatch.com/en/news/history-world-cup-finals/#:~:text=The%20first%20World%20Cup%20final%20was%20played%20on,Iriarte%2C%20and%20Hector%20Castro%20scored%20for%20the%20hosts.

Immediately after the match on 30 July 1930, the Local Organising Committee calculated ticket sales. This internal accounting process yielded an exact number of 68,346 paid spectators. FIFA formally adopted this figure when processing the Uruguayan Football Association's financial and administrative balance sheets in late 1930. It was entered into the official historical ledger to distinguish paid ticket holders from the large unrecorded crowd.

However, other sources cite figures of up to 90,000 or even 100,000 spectators.

The Press Association's World Cup final report gave the attendance as 100,000. The official attendance was subsequently recorded as 68,346 but, with reports claiming that many thousands were turned away from a 90,000 capacity stadium, the 100,000 figure might be more accurate. In any case, the attendance for the biggest football match that had ever been played was not quite as big as had been seen at cup finals and home internationals in Britain.

https://medium.com/soccer-stories/the-so-called-worlds-association-football-championship-3f825e7b10e6

Some sources give the actual attendance as 93,000 (the designated capacity of the stadium for the World Cup).

The 100,000 capacity of the Centario stadium was not completed until 18th July 1930, 5 days after the tournament started. Construction had commenced in February 1930. The organisers limited the capacity to 93,000 for safety reasons.
https://lermitte.be/boek-engels/History%20of%20the%20World%20Cup_%201930%20-%202010%20-%20Clemente%20Angelo%20Lisi.pdf
(at p25 and p34 of 598)

FIFA have now put the number in attendance at 80,000
https://www.fifa.com/en/tournaments/olympicgames/paris2024/articles/uruguay-1924-1928-1930-world-cup