Deadpool made his debut in The New Mutants #98 (Feb. 1991), being a hired assassin to attack Cable and the New Mutants.
His second appearance was in X-Factor, and he then guest appeared in various titles such as The Avengers, Daredevil, and Heroes for Hire, until he got his own mini-series, The Circle Chase, in 1993.
It was then that he became somewhat of a hit, and started developing his own cult following.
Four years later, he got his own ongoing title, becoming an exaggerated anti-hero, which almost poked fun at similar superhero/vigilante comics that were coming out at the time.
It was in his solo series that many traits and trademarks would appear and become developed.
A comic book main character going unpunished for crimes was seen as a moral issue with many. But he gained popularity nevertheless.
Deadpool joined several different versions of himself in the 12-issue Deadpool Corps, including the series prequel, Prelude to Deadpool Corps: Lady Deadpool, Deadpool, Kidpool, Dogpool, and The Champion a.k.a. Championpool.
Deadpool has only died once so far—besides when he was a zombie in the Marvel Zombies comics—and that was in Deadpool #45, Deadpool's official 250th appearance, released in 2015 with several cover variants.
At the time of this writing, Deadpool has appeared in over 1700 titles.
So, if you're a collector of Deadpool appearances, it won't be a cheap collection.
He might not be the most famous character in comic books, but he does get around and has his cult following.