Based on what I could find in the literature, 1 fume event per 5000 flights seems to be the best estimate for frequency. The frequency may be higher for some airlines based on their fleets; I recall Allegiant often making the news for fume events on their MD-80s and A319s. As for hospitalizations it’s hard to say but for chemical exposures it not uncommon to briefly admit someone for supplemental oxygen therapy and monitoring. Most of the fume events I’ve read about have a handful of passengers treated on-scene or at the hospital.