In the 1990s, Venom was easily one of Marvel's single most popular characters, for a pretty obvious reason. He was basically Frank Castle plus Peter Parker, a grimmer, grittier Spider-Man who billed himself as a "lethal protector" and had a mouth full of teeth so long and sharp that even Wolverine thought it was a bit much. He was so popular, in fact, that from 1993 to 1998, there was a new issue of Venom on the stands every single month without missing a beat. It just never managed to get past #7.
Rather than being a single series that ran from #1 to #60, Venom's ongoing adventures were broken out into a staggering 17 different series that ran back to back to back to back to back to back to — well, you get the idea. It started in February of '93 with Venom: Lethal Protector #1 – 6, and ran through stories like Separation Anxiety (four issues in '94 to '95), Sinner Takes All (five issues in '95), License to Kill (three issues in '97), and a dozen other miniseries, plus a handful of specials thrown in for good measure, before finally wrapping up with the appropriately named three-issue Venom: The Finale in January of 1998.
As for why they didn't just keep Lethal Protector going after its first six issues, or at the very least launch a real ongoing once they were a couple years into this whole needlessly complicated experiment, well, that's simple. Everyone knows that a #1 issue is more valuable for collectors, right? And what's better than one #1? Seventeen #1s! Wait, what do you mean that's not how supply and demand works?