Counting Telos and Quick Reads novellas, but not short story collections, Torchwood novels or SJA novelisations (gotta draw the line somewhere, that still leaves 460-odd books!), and giving each author a full credit for co-authored works:

Terrance Dicks leads the pack with 76 – 64 Target novelisations, plus also Timewyrm: Exodus, Blood Harvest and Shakedown from the New Adventures, The Eight Doctors and Endgame from the Eighth Doctor Adventures, and Catastrophea, Players, Warmonger, Deadly Reunion and World Game from the Past Doctor Adventures, and Made of Steel and Return of the Judoon in the Quick Reads series

Next with 17 is Justin Richards: Theatre of War from the New Adventures, System Shock and The Sands of Time from the Missing Adventures, Option Lock, Demontage, The Banquo Legacy, The Burning, Time Zero and Sometime Never… from the Eighth Doctor Adventures, Dreams of Empire, Millennium Shock, Grave Matter and The Shadow in the Glass from the Past Doctor Adventures and The Clockwise Man, The Deviant Strain, The Resurrection Casket and Martha in the Mirror from the New Series Adventures.

Next with 12 are David A. McIntee, Kate Orman, Stephen Cole and Gary Russell. McIntee has written NA’s White Darkness, First Frontier and SanctuaryLords of the Storm, The Shadow of Weng-Chiang and The Dark PathAutumn Mist and PDAs The Face of the Enemy, Mission: Impractical, The Wages of Sin, Bullet Time and The Eleventh Tiger.

Orman’s credits are for The Left-Handed Hummingbird, Set Piece, SLEEPY, Return of the Living Dad, So Vile a Sin and The Room With No Doors of the New Adventures, Vampire Science, Seeing I, Unnatural History and The Year of Intelligent Tigers from the EDAs, the PDA Blue Box and the Telos novella Fallen Gods.

Cole’s name is on EDA’s Parallel 59, The Ancestor Cell, Vanishing Point, Timeless and To the SlaughterThe Shadow in the Glass and Ten Little AliensThe Monsters Inside, The Feast of the Drowned, The Art of Destruction and Sting of the ZygonsFrayed because he wrote it as "Tara Samms".

Christopher Bulis on 11: NA: ShadowmindState of Change, The Sorcerer’s Apprentice, The Eye of the Giant, Twilight of the Gods and A Device of DeathVanderdeken’s ChildrenThe Ultimate Treasure, City at World’s End, Imperial Moon and Palace of the Red Sun.

Steve Lyons also 11: NA’s Conundrum and Head Games, MA’s Time of Your Life and Killing Ground, EDA The Space Age and The Crooked World, PDAs The Murder Game, The Witch Hunters, Salvation and The Final Sanction, and NSA The Stealers of Dreams.

Gary Russell also on 11: include the novel of the 1996 TV movieLegacy, MA’s Invasion of the Cat-People and The Scales of Injustice, EDAPlacebo Effect, PDAs Business Unusual, Divided Loyalties, Instruments of Darkness and Spiral Scratch, and NSA Beautiful Chaos

9: Ian Marter, Gareth Roberts,  Paul Cornell, John Peel and Paul Leonard
8: Lance Parkin and Mike Tucker
7: Trevor Baxendale, Simon Messingham, and Malcolm Hulke
6: Barry Letts, Dave Stone, Jacqueline Rayner, Jim Mortimore, Marc Platt, Martin Day and Nigel Robinson (and I can’t be bothered to link any further)
5: Andrew Cartmel, Andy Lane, Craig Hinton, Gerry Davis, Keith Topping, Lawrence Miles, Paul Magrs, Robert Perry
4: Ben Aaronovitch, Chris Boucher, Colin Brake, David Bishop, Eric Saward, Jonathan Blum, Mark Gatiss, Mark Michalowski, Mark Morris, Nick Walters and Pip and Jane Baker
3: Christopher H. Bidmead, Daniel O’Mahony, Donald Cotton, Ian Stuart Black, John Lucarotti, Jonathan Morris, Lloyd Rose, Peter Anghelides, Peter Darvill-Evans, Peter Grimwade, Philip Hinchcliffe, Philip Martin, Simon Bucher-Jones, and Simon Guerrier
2: Brian Hayles, Dale Smith, Daniel Blythe, David Fisher, David Whitaker, Ian Briggs, Mark Clapham, Michael Collier, Mick Lewis, Paul Scoones, Robert Holmes, Simon A. Forward, Stephen Gallagher (as John Lydecker), Stephen Wyatt, Steve Emmerson, Terence Dudley and Victor Pemberton
1: Alison Bingeman, Andrew Hunt, Andrew Smith, Barbara Clegg, Bill Strutton, Dan Abnett, David Banks, David Lawrence, Eric Pringle, Glen McCoy, Glyn Jones, Graeme Curry, Graham Williams, Iain McLaughlin, James Swallow, Jeremy Hoad, Jon Blum, Jon de Burgh Miller, Jon Preddle, Kelly Hale, Kevin Clarke, Kim Newman, Louise Cooper, Mags L Halliday, Malcolm Kohll, Mark Chadbourn, Matthew Jones, Natalie Dallaire, Neil Penswick, Nick Wallace, Paul Ebbs, Paul Erickson, Paul J. McAuley, Paul Saint, Peter Ling, Rona Munro, Russell T. Davies, Simon A Forward, Simon Clark, Stephen Marley, Tom Arden, Wally K. Daly and William Emms

In case anyone ever asks you.