How much fuel aircraft should carry to complete flight plan?
This may vary elsewhere in the world but according to the FAA FAR Sec. 121.639

Fuel supply: All domestic operations.
No person may dispatch or take off an airplane unless it has enough
fuel--

(a) To fly to the airport to which it is dispatched;
(b) Thereafter, to fly to and land at the most distant alternate airport
(where required) for the airport to which dispatched; and
(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption
or, for certificate holders who are authorized to conduct day VFR
operations in their operations specifications and who are operating
nontransport category airplanes type certificated after December 31,
1964, to fly for 30 minutes at normal cruising fuel consumption for
day VFR operations.