I use 25 flaps for landing.
For 99% of aircraft, yes. Flaps are a necessary tool in landing as they are a key component to controlling speed and altitude during the approach and landing process.
For most of the aircrafts full flaps are required to fly at the landing speed, else you will stall and won’t be able to control the pitch or else you vertical speed will be high breaking backs of the passangers
Landing configuration differs from aircraft to aircraft, your landing weight and runway landing distance available. It won’t always be full, in fact most of the time it doesn’t have to be. For example, yesterday at KDFW RWY 17L in a B77F I only needed flaps 25 and came in at 140kts.
It’s a very broad question. Yes you need flaps, unless you really want to land at a very high speed and have a suitable runway for your aircraft and weight. How much depends on each situation.
Flaps are a very important thing for landing, they reduce speed and increase air resistance so your plane doesnt slam on the ground. Depending on what aircraft you want flaps for most planes all the way down.
Suggest that all pilots have a good read of the FAA Pilot Handbook. Lots of great information giving you all the basic knowledge that you need.
Yes you should use flaps when you are landing. In emergency situations when flaps fail pilots have to land at high rates of speed of less than 200 knots. Normally landing speeds vary from 130-150 knots for most aircraft.
Check out my tutorial for the flaps on the 737-800/900. It applies to almost every aircraft.