There are 18 types in the main games as of 2020
Pokemon is a complex and ever-changing world that has 'evolved' (get it?!) over the years to include more types than were originally introduced in 1996.
But, what is a 'Pokemon type'?
A Pokemon type is basically an elemental power inherent to certain Pokemon that is both strong against some types, and weak to others.
Pokemon are able to use attacks of different types to try and gain an advantage over their opponent. It's a very complicated game of Rock-Paper-Scissors.
When first generation of Pokemon were introduced 1996's Red & Blue versions, there were 15 types:
Bug
Dragon
Electric
Fighting
Fire
Flying
Ghost
Grass
Ground
Ice
Normal
Poison
Psychic
Rock
Water
The second generation of Pokemon were introduced in 1999's Silver & Gold versions. Two more types were added to reduce the dominance of psychic types and improve balance:
Dark
Steel
A new type wasn't added again until Pokemon X & Pokemon Y versions were released in 2013. This new type was to reduce the dominance of dragon types and improve balance:
Fairy