President Reagan signed into law a $20.1-billion air traffic control bill that increases the number of controllers and authorizes airport improvements and equipment. The measure orders the Federal Aviation Administration to have at least 15,900 air traffic controllers on the job by Sept. 30. The FAA has said the current work force totals 15,400 controllers. The legislation also extends for three years an 8% tax on airline passenger tickets, as well as other levies to finance aviation projects. It orders the FAA to require airliners within four years to carry an instrument that can warn of impending collisions. The President also signed bills banning the dumping of most plastic garbage from ships closer than 25 miles to shore, and extending an agreement permitting joint U.S.-Japanese fishing operations in Alaskan waters. Reagan is vacationing in Rancho Mirage, Calif.