Why does Horo call Lawrence Master?
Disclaimer: I'm no expert in Japanese, just a die hard fan of Spice and Wolf.

Holo speaks in a dialect known as "Oiran" that often replaces the common "Watashi" to "Wacchi" among other things. The creator of Spice and Wolf, Isuna Hasekura-sensei made a few adjustments here and there so Holo's dialect is loosely based on it. There has been a bunch of people in the past discussing this here (a reddit post).

Additionally, you can see in an interview with Isuna Hasekura-sensei here where He mentions Holo's unique speech pattern (which is antique in a way), and the fact that this unique way of talking is often lost/corrupted in translation.

When Holo says "Nushi" it roughly translates to "You" when taken into the correct context. Many translators translate the word "Nushi" to mean "Master/Valued Customer", because the word "Nushi" is more commonly used by Courtesans (back in the day) to greet their guests.