Apologies if this annoys proponents of “toposes “.
It appears to me that there are three main candidates for pronunciation, all focusing on the last syllable:
- Top-oy (rhyming with “toy” in British English)
- Top-wa
- Top-oh-ee (less likely, I feel)
So which, if any, is most commonly used? I do see this as an objective question, in the same way that there is a correct answer to “what is the commonly accepted pronunciation of ‘Lie’?”.
The only pronunciation I have ever heard (American) English-speaking mathematicians use is "toe-poy" /ˈtoʊpɔɪ/ (with stress on the first syllable). That is consistent with the pronunciation of the singular as "toe-poce" /ˈtoʊpoʊs/ and the usual English pronunciation of "oi" /ɔɪ/ (except in words that come from French, which topoi does not).
For what it's worth, Wikipedia seems to agree with this pronunciation in American English but gives "top-oy" /ˈtɒpɔɪ/ instead as the pronunciation in British English. (To be precise, it gives the corresponding pronunciations for the singulars in American and British English and then mentions both plural pronunciations without saying which dialect they correspond to.)