In addition to Ford, two more carriers are currently planned or under construction. The second ship in class, USS John F. Kennedy, should join the fleet in 2020. The third, USS Enterprise, should join the fleet in 2025.
The USS Nimitz is planned for retirement in the 2022 to 2025 time frame. If Kennedy and Enterprise join the fleet on schedule, there will be a brief, two year period between 2020 and 2022 when the U.S. will have 12 carriers—a number which, if held to schedule, dovetails nicely with the next presidential election season. After that, Nimitz will fall off the roster and Enterprise will replace USS Eisenhower, dropping us back to 11 carriers again.
What if Trump wanted to make that magic number 12 more durable? He'd have to start construction of a fourth Ford-class carrier ASAP OR defer the retirement of the USS Nimitz.
Twelve carriers doesn't mean that the Navy can put all twelve ships to sea during a crisis, either. The typical rule of thumb is that during peacetime it takes 12 ships to keep four ships at sea at all times. For every one ship at sea, a second is training and being outfitted to go to sea and a third has just returned and is standing down. The number of carriers can surge in a crisis--during the 1991 Persian Gulf War the U.S. Navy should normally have been able to field five carriers at most but managed to surge six to participate in the air war against Iraq.
Each modern carrier probably packs the firepower of at least 15 World War II carriers. The ability of a Nimitz-class carrier to place bombs on target hundreds of miles away with accuracy measured in inches makes each orders of magnitude more powerful than even carriers of the Vietnam War period.
In December 1941, the U.S. Navy had seven aircraft carriers and one smaller escort carrier. Four years later, the Navy had 99 carriers of both types. At the height of the Vietnam War, it had 25 carriers. At the end of the Cold War it still had 14 ships.
The US currently has 9 other smaller aircraft carriers that have helicopters but which could have vertical takeoff planes like the F35B or Harrier jumpjets.
SOURCE - Popular Mechanics, Global Security, wikipedia