With the start of World War I, Porte returned to service in the Royal Navy, which subsequently purchased several models of the America, now called the H-4, from Curtiss. Porte licensed and further developed the designs, constructing a range of Felixstowe long-range patrol aircraft, and from his experience passed along improvements to the hull to Curtiss. The later British designs were sold to the U.S. forces, or built by Curtiss as the F5L. The Curtiss factory also built a total of 68 "Large Americas", which evolved into the H-12, the only American-designed and -built aircraft to see combat in World War I. As 1916 approached, the United States was feared to be drawn into the conflict. The Army's Aviation Section, U.S. Signal Corps ordered the development of a simple, easy-to-fly-and-maintain, two-seat trainer. Curtiss created the JN-4 "Jenny" for the Army, and the N-9 seaplane version for the Navy. They were some of the most famous products of the Curtiss company, and thousands were sold to the militaries of the United States, Canada, and Britain. Civilian and military aircraft demand boomed, and the company grew to employ 18,000 workers in Buffalo and 3,000 workers in Hammondsport. In 1917, the U.S. Navy commissioned Curtiss to design a long-range, four-engined flying boat large enough to hold a crew of five, which became known as the Curtiss NC. The four NC flying boats attempted a transatlantic crossing in 1919, and the NC-4 successfully crossed. It is now on permanent display in the National Museum of Naval Aviation in Pensacola, Florida.

Answer this question "How many employees did his company employ in Buffalo during World War 1?" by extracting the answer from the text above.
18,000 workers in Buffalo