In honor of Slott leaving Spider-Man (and his observation and his observation that the only milestone he had left was to surpass Bendis as the most prolific writer) I looked at the contributions of the other really prolific writers, the guys who had either lengthy or multiple runs of the series. Or both.
Looking at other writers..
Gerry Conway did Marvel Team Up #2-12, 28-37, 52, Amazing Spider-Man 111-149, Annual #23, Giant Sized Superheroes #1, Giant-Sized Spider-Man #3-5, Spectacular Spider-Man #1-3, 137-174, Annual #8-11, Web of Spider-Man #35-36, 47-48, 50-70, Annual #5-6, the Spiral five issue mini-series and ten issues of Renew Your Vows. That's 150 issues, in addition to some graphic novels (Parallel Lives, Way to Dusty Death, Fear Itself.) I'm not counting Carnage, as Spider-Man didn't appear. He would be #3.
Tom DeFalco wrote Amazing Spider-Man 251-263, 265, 268-285, 407-439, Annuals 15-18, 23, 24, '96, '98, -1, Marvel Team Up #96, 106-107, 138, 140, Spectacular Spider-Man #41, 215-229, 254-255, Spider-Man Unlimited #1-6, 15, 18, Amazing Scarlet Spider #1-2, Web of Scarlet Spider #1-2, Webspinners #17-18, the Mysterio Manifesto #1-3, Spider-Man #1/2, 26, Untold Tales of Spider-Man #22-24, the Original Clone Saga mini-series, and Sensational Spider-Man 33.1-33.2 . That's 129 issues of Spider-Man. Obviously, the number goes up by a lot if you're counting Spider-Girl, where Peter Parker is a supporting character. He would be #4. Or maybe #1, with all the Spider-Girl.
David Michelinie wrote Amazing Spider-Man #205, 290-292, 296-352, 359-388, 545, Annual #21-26, 28, Annual '95, Web of Spider-Man #8-9, 14-20, 23-24, 70, Annual #7-8, Super Special #1, Spectacular Spider-Man #173-175, 220, Annual #11-12, Super Special #1 (19911995), Spider-Man #35, Super Special #1. That's 124 issues. He would be #5.
Stan Lee did ASM 1-100, 105-111, Annuals 1-5, Spectacular Spider-Man Magazine #1-2, and Marvel Superheroes #14 (an inventory story with Ross Andru.) That's 115 (5 of which were deluxe-size.) There's obviously Amazing Fantasy 15, but that's technically not a full issue. He did write a few assorted one-shots (Spider-Man/ Kingpin; To The Death, Amazing Spider-Man Annual #19 with Tom DeFalco as co-plotter. He would be #6.
JM DeMatteis wrote Marvel Team Up #101, 111-112, 114-133, Kraven's Last Hunt, Spectacular Spider-Man #-1, 178-203, 217, 223, 241-257, Annuals 13-14, Amazing Spider-Man 223, 389-406, Annual 24, Spider-man 37-40, 51, Web of Spider-Man 117, Webspinners 1-3, Soul of the Hunter, Lost Years 1-3, Redemption 1-4, and two Spider-Man/ Batman specials. That comes to 114 issues. He would be #7.
Bill Mantlo did The Amazing Spider-Man #181, 222, 237, Annual #1011, 17, Marvel Team-Up #3851, 5356, 72, 134135, 140, Annual #1, 6, and The Spectacular Spider-Man #6, 910, 1215, 1734, 3640, 42, 53, 6189, 104, 120, Annual #1, 4. That comes to 97 issues. He would be #8.
In some cases, writers shared duties on an issue. Tom DeFalco often wrote over someone else's plot. I tried not to count back-up stories, although that would also inflate numbers.