From various sections of the RollerCoaster Tycoon Wikipedia article
RollerCoaster Tycoon received two expansion packs: Added Attractions (released in the US as Corkscrew Follies) in 1999, and Loopy Landscapes in 2000. Two special editions were released: RollerCoaster Tycoon Gold/Totally Roller Coaster in 2002, which contained the original game, Added Attractions/Corkscrew Follies, and Loopy Landscapes; and RollerCoaster Tycoon Deluxe in 2003, which contained the content in Gold plus more designs for the different customizable rides.
There are 21 scenarios included with RollerCoaster Tycoon, as well as 30 more in the Corkscrew Follies expansion pack, and another 30 in the Loopy Landscapes expansion pack, totaling to 81 scenarios if the whole set is installed. There are also 3 promotional scenarios released with magazines and 11 official scenarios created by Hasbro for competitions, as well as 3 real amusement parks and 1 extra park that are available in the Deluxe edition
RollerCoaster Tycoon features 22 scenarios, with the first five open from the start. Successfully completing a scenario will unlock the next one. Additionally, Mega Park is unlocked when all 21 scenarios are completed. This scenario has no objective (except "Have Fun!"), but allows the player to build on nearly the entire map, and eventually has all attractions available to build. This is the closest to a "sandbox" park, except the player does not have an unlimited amount of money to work with, only a $50,000 loan.
Some scenarios are based on real parks. For example, 'Katie's Dreamland' (Katie's World in the US Version) is based on Lightwater Valley, complete with that park's signature The Ultimate roller coaster (The Storm).
There were two official scenarios available for download from Atari's website: Fort Anachronism and Alton Towers. Alton Towers was included with the Loopy Landscapes expansion with Heide-Park and Blackpool Pleasure Beach and was updated to take advantage of the new game components. Both scenarios are also included as part of RollerCoaster Tycoon Deluxe. The UK edition of RollerCoaster Tycoon Deluxe contained Blackpool Pleasure Beach in place of Fort Anachronism.
Source: Wikipedia, "[en.wikipedia.org/wiki/RollerCoaster_Tycoon RollerCoaster Tycoon]", available under the CC-BY-SA License.