In case you are not familiar with the game Six Degrees of Kevin Bacon it is based on the "six degrees of separation"idea that any two people on Earth are, on average, about six acquaintance links apart. That idea turned into this popular game. People challenge each other to find the shortest path between an arbitrary actor Kevin Bacon.
David Tennant was in L.A. Without a Map with Julie Delpy and she was in The Air I Breathe with Kevin Bacon. This gives David Tennant a Kevin Bacon number of 2. You can also link him this way - David was in Harry Potter and the Goblet of Fire with Gary Oldman and Gary Oldman was in Murder in the First with Kevin Bacon.
Just for fun since David loves the West Wing so much here is his Six Degrees of Separation from the main cast:
1 degree from Stockard Channing - they were both in Bright Young Things
2 degrees from Dule Hill - Dule who was in Sugar Hill with Joe Dallesandro who was in L.A. without A Map or Dule Hill was in Sexual Life with James Legros who was in L.A. Without a Map
2 degrees from Allison Janney who was in Chumscrubber with Jason Isaacs who was in HP and the Goblet of Fire - or Allison Janney was in Piccadilly Jim with Pam Ferris who was in Nativity 2 also you can use Piccadilly Jim with Hugh Bonneville who was in Glorious 39 or Allison was in The Hours with Toni Collett who was in Fright Night
2 degrees from Rob Lowe who was in Tommy Boy with Dan Aykroyd who was in Bright Young Things or Rob Lowe was in I Melt with You with Jeremy Piven who was in Pirates! Band of Misfits or he was in Oxford Blues with Pip Torrens who was in St. Trinian's 2
2 degrees from Janel Moloney who was in Safe with James Legros who was in L.A. Without a Map
2 degrees from Richard Schiff who was in Made in Dagenham with Richard Lloyd-Pack who was in HPGOF or Richard was in Johnny English Reborn with Clara Paget who was in St. Trinian's 2
2 degrees from Martin Sheen who was in JKF with Gary Oldman who was in HPGOF
2 degrees from John Spencer who was in Twilight with Stockard Channing who was in Bright Young Things
2 degrees from Bradley Whitford who was in Decoding Annie Parker with Alice Eve who was in The Decoy Bride or Bradley was in Bottle Shock with Alan Rickman who was in HPGOF or he was in My Fellow Americans with Dan Aykroyd who was in Bright Young Things