How many UFO sightings have been in the US?

There are no real statistics for UFO sightings. You can only compile information for sightings that find their way into official records. Many UFOs are eventually identified and therefor no longer UFOs.

When a person anywhere in the US sees something flying that they can't identify, who are they going to report it to; the military, the local airport, 911, or the local news. Each of those people may choose a different option. Who they contact and what becomes of that report can't guarantee that it will find its way into an official record from which statistics can be compiled.