How many earthquakes happen every year?
According to the US Geological Survey there are approximately 1,000,000 earthquakes around the world every year (although the vast majority of these are so small they can only be detected by sensitive scientific equipment).
Of these 1,000,000, approximately 1/10 are large enough to be felt by humans. This means that on any day there will be on average 275 earthquakes around the world that can be felt.
Perhaps even more surprisingly, on average there are around 100 earthquakes a year that are large enough to cause damage (however they often don't because they occur away from inhabited areas). This means there is a little under a 1 in 3 chance of there being a "large" earthquake on any given day.
The British Geological Survey have published some data on the annual average frequency of earthquakes of varying magnitudes:
Description : Magnitude : Frequency per year
Great : > = 8.0: 1
Major : 7.0 - 7.9: 17
Strong : 6.0 - 6.9 : 134
Moderate : 5.0 - 5.9 : 1319
Light : 4.0 - 4.9 : 13,000 (estimated)
Minor: 3.0 - 3.9 : 130,000 (estimated)
Very minor : 2.0 - 2.9: 1,300,000 (estimated)