Ash has completed two Grand Trials thus far: Hala's and Olivia's. He still needs to complete two more: Nanu's and and Hapu's.
The number of trials differs between the games and the anime. In the games, there's one Island Trial in Melemele, three in Akala, two in Ula Ula, and one in Poni (though in USUM, that number increases to two). In the anime, while Ash has completed the single Melemele trial, he only had to beat one Island Trial in Akala to face Olivia: Totem Lurantis.
A recent poster for the upcoming season of the anime has implied Ash will also be doing his next Island Trial in Ula Ula during the entire Ultra Beast arc:
If you look to the top right:
Yep, Nanu and Acerola will be showing up in the coming arc.
How many trials has Ash done? And how many does he still need to complete? I forget how many trials there actually are, plus haven't beat Sun and Moon yet.
I bet Ash will only need to do the Ghost Trial before facing Nanu.