The range of an air launched anti-air missile depends on both the missile itself as well as the speed and altitude it's launched: the faster and higher, the more initial energy and the farthest it can go. On top of that, we have the range at which the launching aircraft can detect the hostile.
So, yes, an airliner can hold several dozens of missiles (maybe even a hundred, easily), as well as a radar capable of detecting foes hundreds of km away. It's also subsonic and built to work in a rather low ceiling. So a modified AWACS is defending a certain area with it's BVR missiles. It's flying at match 0.9 at 10,000 meters. Bandits approach the area - supersonic jet fighters. Now, the AWACS radar is more powerful than the smaller, yet advanced, radar the fighters at carrying. But the AWACS it's also huge and thus, has a high radar signature. The incoming 4.5+ gen fighters aren't stealth, but they still have a small RCS. Let's say it evens out and they detect each other at a range of 150km. Assuming all aircraft are using similar missiles, the fighters making a dash at match 2.2 at 21,000 meters will have a range advantage over the slower, huge target, flying 11,000 meter below them.
And modern fighters can carry a lot of missiles too. So you're better off with four supersonic fighters carrying, in total, the same amount of missiles the huge subsonic missile truck has.