What is the generally accepted pronunciation of “topoi”?

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.)

Topoi is the Greek word "$\tau \acute o \pi oi$", see wikipedia, i.e. the plural of $\tau \acute o\pi o\varsigma$. So the pronunciation is accordingly (I have only heard "toe-poy" so far in English, and we said it this way in school, where the teacher in Greek said it was the original pronunciation in ancient Greece.).

It is the plural of the word topos which stems from Greek (τόπος in Greek) which is indeed topoi (τόποι). In that case, it's pronounced like to-pee /ˈto.pi/. You give emphasis on the o (that's what the tonos ΄ symbol means over the o). Check here.