Well don't worry too much about the semi auto modes you shoot in just yet because shooting aircraft is for the most part all about how you pan. I am using the two priority modes at the moment rather than manual too as I feel that now I have had plenty of experience shooting manual, I can lay back a bit and rely on the camera, knowing full well that I can spot when the camera isn't making the decisions I want it too. For aviation photography there are one or two pointers I can give you that may help though.

The first is to try and maintain an aperture of around F.8 or narrower. This is known as the sweet spot for aircraft as F.8 seems to keep the aircraft sharp and the photograph will have good depth of field for the subject. If you look at most of my photographs the exifs will mostly have my aperture set between F.8 and F.11

As for camera modes, if you are using the priority modes you want to use shutter priority when shooting aircraft with propellers as this will allow you to set the shutter to 1/320 or slower meaning that the photograph will have good propeller movement and therefore look dynamic. For jets use aperture priority as you essentially want to use a shutter as fast as you can whilst maintaining that aperture of F.8 so Aperture priority is perfect for that.

When shooting aircraft you also also have to remember that the brightness of the sky behind the aircraft can trick your cameras light meter causing either ghosting of the aircraft to be far too dark. It isn't therefore sensible to just rely on the camera to get the exposure perfect. It also doesn't help that light is often changeable so you can't always just set the camera up and leave it that way. The way I do it is to look for something to meter off that will give me similar lighting to the aircraft. Airshows generally have speakers up on poles that can be used to meter off and because they are up high and will have sky behind them if you shoot them. So basically if you take a test shot of the speaker and it comes out either too dark or too light you can then adjust your exposure bias accordingly to hopefully get the correct result for the aircraft.

Anyway I hope all of that is in anyway of help. As for how many decent shots you come home with I couldn't say for sure as it very much depends on your skill level and your own personal standards. After all everybody has different standards and those standards will depend on how long and how advanced you are at shooting. However what I can say is that for me I would take something like 1500-2000 shots per airshow, this for me would usually break down into around 40-80 shots that I would be happy uploading, with around 300 altogether being good or acceptable to me. The rest are either blurry, badly cluttered or with the aircraft only partly in frame.

The main thing though is that you enjoy it. It doesn't matter that much if you get bad shots because that's all part of the fun. We are all learning no matter how advanced we are.