Here "you" is singular and refers to Abram, but Abram represents his descendants. Alternate translation: "I will start a great nation through you" or "I will make your descendants become a great nation" (See: [[rc://en/ta/man/jit/figs-metonymy]])
The words "to other people" are understood. Alternate translation: "you will be a blessing to other people" (See: [[rc://en/ta/man/jit/figs-explicit]])