With #557, the Amazing Spider-Man passed the Fantastic Four to become the Marvel comic with the highest issue number. It did this because it has gone weekly, of course, but it's a little more complicated than that. Back in the early sixties, Spidey was actually the third Marvel super-hero series to get its own book. The FF was first with its premiere issue cover-dated November 1961. The Hulk was next with its #1 dated May 1962 but that series was cancelled after issue #6, cover-dated March 1963, the same cover-date as Amazing Spider-Man #1. Spidey was bi-monthly for the first four issues. By the time it settled into its monthly routine the Fantastic Four was fourteen issues ahead of it. That's what I remember as a kid in the sixties. If you wanted to know the current FF issue number, look at the latest Spidey and add fourteen. If you wanted to know the latest Spidey number, look at the FF and subtract fourteen.
So, then the Fantastic Four was the highest issue number of the silver age Marvel heroes, then? Of course not. Before the super-hero books, Marvel (Atlas) was publishing monster/horror/suspense anthology titles. These titles were taken over by the various heroes during this time and all had higher issue numbers than the FF. You all know this. Ant-Man (Giant-Man) took over Tales to Astonish, was joined by the Hulk and replaced by Sub-Mariner. Iron Man and Captain America were in Tales of Suspense. Dr. Strange and Nick Fury (but first, the Human Torch) were in Strange Tales. Thor had Journey Into Mystery all to himself, which really confused things when Marvel quietly changed the title to The Mighty Thor with #126 (March 1966), making it look like Thor was the first silver-age Marvel hero. Something a number of us believed back then since there weren't any back issues around to tell us differently. When the "two-in-one" hero books were separated, the numbering of the anthologies remained. In 1968, Sub-Mariner, Iron Man, and Nick Fury all started with issue #1 but the Hulk, Cap, and Doc Strange retained the numbers from Astonish, Suspense and Strange Tales. If we pull a random Spidey issue from around that time... say, ASM #67, December 1968... and look at the Mighty Marvel Checklist, we find plenty of series with lower numbers (Silver Surfer at #3, Not Brand Echh at #11, Avengers at #58, X-Men at #50, DD at #46, etc.) but also the FF at #81 (plus 14, remember?), Cap at #108, Hulk at #110, Thor at #158, and Dr. Strange at #175. Note in that same issue (which was selected at random, believe me) that the Society for Comic Art Research and Preservation chose Spider-Man as the best adventure comic (single character). But numberwise, Spidey was comfortably in the middle of the pack.
Does any of this mean anything? Not really. As we've seen, having a high number doesn't indicate longevity. As we later discovered when Marvel started rebooting all their series, including Amazing Spider-Man, FF, DD, Thor, Hulk, Avengers and so on and so on, having a low number doesn't denote newness either. Back during that period, the Marvel series with the highest number was Uncanny X-Men with Wolverine in second, as I recall. That really demonstrated how arbitrary it all is and proved that it doesn't matter what number is on the cover. Except... didn't it somehow feel wrong when Amazing Spider-Man's numbering was reset back to #1? And didn't it feel great to have it return to #500? (The FF also returned to #500, hence the vying between the two series for the highest number, but what with twice-monthly events and such things, the "plus fourteen" rule was sadly a thing of the past. And, Spidey's weekly status overcame the rest of the gap.) So, who cares what the number is but doesn't it feel kind of neat to have Spidey now lead the way with Marvel's highest numbering? And considering, forty years after that Comic Art Research vote, Spidey is still often chosen as "the best adventure comic (single character)", doesn't it feel like this is the way it should be?