#5 (tie): Jason. Castro, Frasor, Grilli, Hammel, Kipnis, Kubel, Marquis, Vargas. There are also two Jaysons...Nix and Werth.
#5 (tie): Justin. Masterson, Maxwell, Morneau, Ruggiano, Sellers, Turner. Upton, Verlander, Wilson. (Note that none of them have a last name starting with A through L.)
#4: Josh. Beckett, Collmenter, Donaldson, Edgin, Fields, Harrison, Johnson, Reddick, Rutledge, Wilson. All spelled the same way.
#2 (tie): Matt and Ryan. So Matt wins on an alphabetical tiebreaker...unless you do it in reverse order. The Matts are Adams, Albers, Belisle, Cain, Carpenter, Dominguez, Guerrier, Harrison, Kemp, Latos, Reynolds, Thornton, Tuiasosopo (does he have a fake girlfriend?) and Wieters. Ryan (last year's winner) includes Braun, Cook, Dempster, Doumit, Flaherty, Hanigan, Howard, Jackson, Ludwick, Mattheus, Pressly, Vogelsong, Webb and Zimmerman.
#1: Chris. Capuano, Carter, Coghlan, Davis, Denforia, Getz, Iannetta, Johnson, Leroux, Medlen, Nelson, Parmalee, Perez, Resop, Sale, Stewart, Valaika, Volstad, Young.
John would have been 3rd if you counted all the variations (John, Jon, Jonny, Johnny, Jhonny). There are also 20 players who use initials: AJ Burnett, AJ Ellis, AJ Pierzynski, AJ Ramos, AJ Griffin, AJ Pollock, BJ Upton, CC Sabathia, CJ Wilson, JC Gutierrez, JD Martinez, JP Arrencibia, JJ Hardy, JJ Hoover, JJ Putz, JP Howell, RA Dickey, TJ McFarland. AJ is clearly the most popular combo. 18 of the 20 have J as at least one of the initials.
Apropos of nothing (stolen from Nick Cafardo of the Boston Globe)...Homer Bailey is not a good name for a pitcher. JJ Hoover, also a pitcher, should be a 3rd baseman with that name. JB Shuck makes it sound like he doesn't care. Jake Peavy sounds like someone who's always in a bad mood. Aaron Harangue sounds like someone who just won't shut up. Mike Leake sounds like he needs to excuse himself for a moment. Justin Turner sounds like someone who needs to wait in line. Doug Fister? Make up your own punch line.
Oh, on the Tom, Dick and Harry front...there is one Tom, but no Dicks and no Harrys.