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.