It’s kind of a meaningless question, because vocabulary is productive; but to Vasiliki Baskos’ answer I will add these figures from non-Modern lexica:
- 19699 from the Liddell Scott lexicon, 2045 from the LSJ supplement; but LSJ does not separate out derived words very well
- 28405 for the DGE Diccionario Griego-Español, which includes proper names
- 23487 from the Papyros dictionary, which encompasses the Dimitrakos dictionary as a dictionary of all periods of Greek
- 11564 from Trapp’s Lexikon zur byzantinischen Gräzität, which mostly includes Byzantine words left out by other dictionaries.
- 5577 from the Kriaras dictionary of Mediaeval Vernacular Greek Επιτομή Λεξικού Κριαρά
- Triantafyllidis’ dictionary of Modern Greek, as Vasiliki Baskos mentioned, is at 8233
And that’s not to mention the Academy of Athens’ Historical Dictionary of Modern Greek (the Greek dialect dictionary), or the serious gaps in Modern Greek lexicography surrounding literary Greek. I don’t have a count for alpha there.
Alpha is a good letter to ask about, btw. Trapp is up to tau, Kriaras is at rho, DGE is at epsilon, and the Academy is at delta. Lexicography is about the long game…