About 1 every 5 years are the number hinted at by David in that they find them around about 1 every 5 years on average. Though he didn't say he was talking about where extra-terrestrial space craft only none human exotic which could mean other things then extra-terrestrial.but Earth seems to be a target for crashed Alien craft because, according to some, we have had a lot.
It all comes down to numbers which we have nothing to really go on. If we go with the 200,000+ UFO reports and assume this means there are 200,000 visits. I know, this is an unrealistic number. But lets pretend everything is true and the amount of visits a year are high. Then 5 to 10 crash over a 50 year timeframe from a high umber of visits is not that unreasonable. It all depends on the ratio of crashes to visits. If they have only visited us 500 times over 50 years then yeah those 12+ crashes seem rather high. That's if aliens are even visiting which I am still not convinced about. As far as I recall David never mentioned extra-terrestrial space craft.