How many UFOs have crashed?

Never. For one thing "UFO" does not mean "alien spacecraft"; it means "I see it up there but I myself don't know what it is". Crafts may crash, if they exist, but "I don't know what it is-ses" can't.

I am aware of no authoritative source that claims the Tunguska object changed course. I am aware of no authenticated photos of the object. As no wreckage was found, it appears that a cometary fragment exploded under the stress of Earth's atmosphere. Comets are largely ice and dust. Large amounts of dust caused bright sunsets over large portions of the Earth. And of course, any ice would have melted in the 20 years it took the government to send a research party.

The possibility exists that the mid-air explosion of a UFO may have triggered the massive Tunguska (Event) in the wilds of Siberia in l908. Eyewitnesses report seeing an unidentified object reverse direction- well against the prevailing winds- and tribesmen living ( back to nature) would recognize shifts in the wind, etc for what they were, shortly before the explosion-which occured at an altitude of about l5.000 feet or slightly under three miles up. a meteor could not have done this, nor an asteroid fragment, space rock, etc. this was something intelligently guided- and massive. There is some evidence- and by accident a Railroad photographer taking a picture of a loco motive on a siding- prior to being coupled to the Trans-Siberian Express, got a picture of an unknown object on his large -frame 4X5 or 8Xl0 plate! the negative and print were immediately confiscated by the IKGB who in due time- turned it over to the KGb when the Reds assumed power in l9l8. Almost certainly this impact- if that is the Word was the result of a UFO crash or malfunction- not a natural object. hence the secrecy.