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$\begingroup$The joint math meeting comes to mind. But like most other examples, it is numbered by calendar year, not by iteration. It seems that it exists since 1989, so pretty long but not close to 100 iterations.$\endgroup$
$\begingroup$It does seem like it would help to have something that meets more than once a year (such as the SLC, AAA, or PSSL already mentioned here), both because that means that using the year is ambiguous and because the number increases more quickly.$\endgroup$
$\begingroup$@ChristianStump "Many more people visit MO, so I should post here rather than in the appropriate site" does not seem a sound argument to me. You could use it to justify all sorts of spam and off-topic content.$\endgroup$
There’s the AAA (Arbeitstagung Allgemeine Algebra), Workshop on General Algebra, which has been running for over 50 years (since 1971). It takes place all around Central Europe several times a year, every year, and it’s currently up in AAA105 to be held in Prague (Czech Republic).
In the aftermath of World War I, at the insistence of the Allied
Powers, the 1920 ICM in Strasbourg and the 1924 ICM in Toronto
excluded mathematicians from the countries formerly part of the
Central Powers. This resulted in a still unresolved controversy as to
whether to count the Strasbourg and Toronto congresses as true ICMs
... As a consequence of this controversy, from the 1932 Zürich
congress onward, the ICMs are not numbered.
The British Mathematical Colloquium is the largest pure mathematical conference to be held annually in the UK. It has been held every year since 1949, usually around Easter, though some of the earlier meetings were in September.
$\begingroup$OP asks for international meetings; I don't have a specific definition of the term, but the fact that this conference is called "British" suggests that it is limited to one country.$\endgroup$
$\begingroup$@FedericoPoloni At least today the participants and speakers of the meeting seem to be from all over the world. Doesn't that make it an international conference?$\endgroup$