Landing With Some Fuel

What do most pilots (commercial) try to land with fuel wise....in other words how much fuel does a pilot try to have left over?
2,000 lbs? (909 kgs)
5,000 lbs? (2272 kgs)
Also do pilots have an estimated fuel ammount and if they feel the need ask for more, or do they have a fuel planner
2,000 lbs? (909 kgs)
5,000 lbs? (2272 kgs)
Also do pilots have an estimated fuel ammount and if they feel the need ask for more, or do they have a fuel planner