There are relatively few known portals and keys, though. The vast majority of the cities doors are unknown and temporary. It's certainly not feasible to use the city as a multiversal highway for travel and trade. Outside of the few known permanent portals, the vast majority of which go to outer planes, you'd have to get very lucky to find one getting you anywhere remotely close to where you want to go.
Well, the boxed set uses the term "thousand pathways" (in a poetic sense of the word
thousand), and In The Cage: A Guide to Sigil says that the city is "lousy with portals". They definitely give the impression that there are a
lot of portals, and that the people who use them aren't necessarily all that choosy about where they open up to. And there
are people who record what portal goes where and what key it requires (because of course there's spells that determine just that) with varying degrees of accuracy and truthfulness. Also, don't forget that in addition to permanent and temporary portals, there's shifting portals, where one end is permanent and the other end moves around.
However, what's important is that the books themselves describe Sigil as a hub for travel and trade. There are
enough permanent and shifting portals to allow for that. The books note that
only the DM is capable of making portals and should use them both liberally and carefully as plot devices, and warns the DM against making a key either too common or too rare, because both can wreck adventures--and the books
also mention that there are shops, plural, in Sigil that do sell nothing
but gate keys, which means there are more than enough portals to support that type of business. And all of this together means that there are
exactly as many available portals as the DM feels is needed for Sigil to remain a hub of trade, travel, and tourism, unless the DM wants all the portals to go away or act weirdly for plot reasons.
Planescape is very meta at times.
Also, canonically in-game, there are plenty of merchants and middlemen who go forth and buy stuff to sell in Sigil (even though one of those merchants apparently has gotten sucked into Ravenloft at some point between the publication of Uncaged: Faces of Sigil and the publication of VRGtR, possibly for the heinously evil crime of writing his entry in Uncaged in an illegible script font). Nobody is expecting Bob the farmer to actually travel to Sigil to sell his wares.