Exactly 2 mol of ammonia react with 1 mol of acetic anhydride. The products are

The first $\mathrm{NH}{}_3$ attacks and the acetate leaving group departs, giving acetamide and acetic acid. The second mole of $\mathrm{NH}{}_3$ is a base and converts acetic acid to ammonium acetate, so the products are acetamide and ammonium acetate.