How many UFO sitings since year 2000?

That is a statistic that doesn't exist; you can only compile information for sightings that find their way into official records.

In other Words, when a person anywhere in the world sees something flying that they can't identify, who are they going to report it to; call 911 or the military, the local airport, 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.