During your journey of
The Galar Region, you will only obtain on
Master Ball, the Poke Ball that guarantees a capture. Actually, you won't receive it until after you have concluded the Main Story, receiving it from
Professor Magnolia to begin the
Post Game. Outside of this Master Ball, you don't receive another within the game. There are ways to obtaining more Master Balls, specifically by using
Cram-o-matic on the
Isle of Armor, but the only guaranteed one you receive is that of the aforementioned Post Game one. That is, until you reach the
Crown Tundra.
That's right! Unlike the
Isle of Armor, you actually receive an extra
Master Ball in the
Crown Tundra. And if that's not enough, you don't really have to do much to obtain it.
All one must do to obtain this Master Ball is battle Peony after arriving to the Crown Tundra, participate in the initial Dynamax Adventure, and then follow Peony to his base camp cabin in Freezington. It is there that Peony will give you the Master Ball.