Currently there are 18 Trials in Alola, 1 for every type. Here is a list:

Verdant Cavern (90): Normal
Iki Town (40): Fighting
Brooklet Hill (28): Water
Wela Volcano Park (91): Fire
Lush Jungle (45): Grass
Ruins of Life (74): Rock
Hokulani Observatory (39): Electric
Thrifty Megamart (83): Ghost
Malie City (47): Dark
Vast Poni Canyon (89): Dragon
Seafolk Village (76): Fairy
Exeggutor Island (31): Ground
Po Town (59): Bug
Mount Lankakila (54): Ice
Ten Carat Hill (82): Flying
Haina Desert (32): Psychic
Mount Hokulani (53): Steel
Ancient Poni Path (22): Poison

Each Trial has 3-4 Pokemon you have to pass to complete, between each Pokemon you are able to !use a different one to prepare for the next Pokemon.

To start a Trial, go to a location in Alola that has a Trial that’s listed above and do !trial to start it. Once you beat the first trial Pokemon, you can !use a different Pokemon if you want, and then do !trial to continue to the next Trial Pokemon and so on until you beat the last one. Once you beat the last Pokemon and do !trial you will get a Z Crystal which can be used to power up a move into a Z move.