maybe look in your research screen as per below screen, the value should be 0 strikes, if you have "1" instead of, then every aircraft upgrade will increase the numbers of strikes available. eg Lvl 5 aircraft will have 5 strikes, maybe this is your case.
There is more than one way to use research upgrades to increase strikes, and they will act in a cumulative way, so it might be best to either cap some of the levels of research, remove the ability of some to increase strikes, and/or reduce this with some by having the number of strikes increase by 0.5 per level rather than 1.