Oh boy, my database loves questions like this
OK. I can't guarantee this is complete, but it's a ton of them. Includes mentions as well as appearances:
John, no last name -- Scandal in Bohemia, occupation: coachman
John, no last name -- Twisted Lip, occupation: coachman
John, no last name -- Missing Three-Quarter, occupation: butler
John Baskerville -- Hound of the Baskervilles, occupation: not given
John Clay -- Red-Headed League, occupation: bank robber
John Clayton -- Hound of the Baskervilles, occupation: cabbie
John Douglas -- Valley of Fear, occupation: secret detective
John Ferrier -- Study in Scarlet, occupation: colonist
John Hardy -- Empty House, occupation: not given
John Vincent Harden -- Solitary Cyclist, occupation: millionaire
John Hare -- Scandal in Bohemia, occupation: actor
John Hebron -- Yellow Face, occupation: lawyer
John Horner -- Blue Carbuncle, occupation: plumber
John Hones -- Study in Scarlet, occupation: colonist
John Mason -- Shoscombe Old Place, occupation: horse-trainer
John Hector Macfarlane -- Norwood Builder, occupation: lawyer
John Mitton -- Second Stain, occupation: valet
John Hopley Neligan -- Black Peter, occupation: not given
John Openshaw -- Five Orange Pips, occupation: not given
John Rance -- Study in Scarlet, occupation: police constable
John Scott Eccles -- Wisteria Lodge, occupation: not given
John Sholto -- Sign of Four, occupation: army major
John Straker -- Silver Blaze, occupation: horse-trainer
John Swain -- Five Orange Pips, occupation: not given
John Turner -- Boscombe Valley, occupation: landowner
John Underwood -- Study in Scarlet, occupation: milliner
John Warner -- Wisteria Lodge, occupation: gardener
John H. Watson -- AKA James, occupation: liar