How many commercial passenger aircraft suffer tail strikes in a given year?
A preliminary answer might be the results of searching incident reports at AVHerald

Accident    Ryanair B738 at London on Jul 29th 2014, tail strike on landing
Incident    Ryanair B738 at Alicante on Mar 27th 2013, tail scrape on takeoff
Report      Jazz DH8C at Gaspe on Sep 10th 2012, hard landing and tailstrike
Incident    Smartlynx Estonia A320 at Tallinn on Jul 31st 2014, tail strike on landing
Accident    Swiss A321 at Geneva on Mar 9th 2014, tail strike on landing
Incident    ANA B763 at Tokyo on Mar 13th 2014, suspected tailstrike on departure
Accident    TAROM B733 at Bucharest on Apr 4th 2011, tail strike
Accident    Lufthansa A333 at Chicago on Mar 5th 2013, undetected tailstrike on takeoff
Incident    Transavia B738 at Amsterdam on Apr 29th 2013, suspected tailstrike
Accident    Aeromexico B762 at Madrid on Apr 16th 2013, tail strike on takeoff
Accident    Titan B733 at Chambery on Apr 14th 2012, tail strike
Accident    Sata A313 at Ponta Delgada on Mar 2nd 2013, tail strike on landing
Report      Jetstar A320 at Sydney on Feb 6th 2012, thrust lever asymmetry during takeoff and early rotation
Accident    China Cargo B772 at Copenhagen on Apr 17th 2011, tail strike on go-around
Accident    Austrian A321 at Manchester on Dec 23rd 2011, tail strike during go-around
Incident    Air Atlanta A306 at East Midlands on Jan 10th 2011, tailstrike on landing, go-around with open thrust reverser
Incident    Air Atlanta A306 at East Midlands on Jan 10th 2011, tailstrike on landing, go-around with open thrust reverser
Accident    Emirates A345 at Melbourne on Mar 20th 2009, tail strike and overrun on takeoff
Accident    Gol B738 at Porto Alegre on Nov 16th 2011, tail strike on landing
Report      Lufthansa MD11 at Mumbai on Jun 11th 2011, tailstrike on landing
Accident    China Cargo B772 at Copenhagen on Apr 17th 2011, tail strike on go-around
Report      Kam Air DC86 at Manston on Aug 11th 2010, tail strike on takeoff
Accident    TUIFly B738 at Tenerife on Mar 27th 2011, rejected takeoff due to tailstrike
Report      Tunisair B736 at Paris on Oct 10th 2008, rejected takeoff after rotation
Accident    United Airlines B744 at Sydney on May 7th 2010, tail strike on takeoff
Accident    Northwest A320 at Denver on May 4th 2009, tail strike on landing
Incident    Austrian B763 at Beijing on Jan 29th 2010, tailscrape on takeoff
Accident    Northwest A320 at Denver on May 4th 2009, tail strike on landing
Report      Ryanair B738 at Dublin on Sep 11th 2008, tailstrike on takeoff
Crash       Avient Aviation MD11 at Shanghai on Nov 28th 2009, overran runway on takeoff
Report      Thomas Cook A332 at Montego Bay on Oct 28th 2008, mislaid manual leads to wrong takeoff speeds
Report      Thomson Airways B763 at Manchester on Dec 13th 2008, tail strike
Incident    PIA B743 at Lahore on Jun 23rd 2009, suspected tail strike leads to discovery of tail strike
Incident    Australian Air Express B463 at Brisbane on Oct 23rd 2008, tail strike on hard landing
Accident    Emirates A345 at Melbourne on Mar 20th 2009, tail strike and overrun on takeoff
Accident    Emirates A345 at Melbourne on Mar 20th 2009, tail strike and overrun on takeoff
Report      Aigle Azur A321 at Algiers on Jan 8th 2008, tailstrike during landing
Incident    Pegasus B738 at Cologne on Dec 20th 2008, tailstrike on takeoff
Report      Flybe DH8D at Paris on May 28th 2008, tailstrike
Report      My Travel A320 at Kos on Jul 5th 2007, very hard landing
Incident    Cargo B B742 at Brussels on Oct 27th 2008, tail strike on takeoff
Incident    Ryanair B738 at Dublin on Sep 11th 2008, tail strike on takeoff
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Incident    Comair (SA) B732 at Cape Town on May 3rd 2008, tail strike
Incident    Ryan Int B738 at Milwaukee on Mar 14th 2008, tailstrike on takeoff
Accident    RyanAir B738 at Stansted on Feb 18th 2008, tailstrike on takeoff
Accident    RyanAir B738 at Stansted on Feb 18th 2008, tailstrike on takeoff
Incident    Delta B767 at Moscow on Jan 26th 2008, tailstrike on takeoff
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Incident    LaudaAir B738 at Linz on Oct 2nd 2007, tailstrike at about taxi speed
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Accident    Swiss RJ100 at London City on Aug 18th 2007, tailstrike on landing
Incident    RyanAir B738 at Dublin on Mar 20th 2007, tailstrike during takeoff


You need to check some of these, some of them mention tailstrike where none occurred (e.g. inapppropriate pilot action due to fear of tailstrike)

You may need to define tailstrike, maybe a controlled flight into terrain sometimes involves a tailstrike but wouldn't be categorised as such.