Airstones are mostly useless in today's hobby. Notice I said mostly.
They are not used to inject air; they are 1980s way to provide circulation in the tank to bring water from the bottom to the surface so that it can exchange CO2 with O2 from air. This exchange happens at the very top of the water column (surface). Water needs to be circulated in the tank so that the bottom part of the tank doesn't become anaerobic and stagnant with little to no oxygen.
Welcome to 21st century, we not have powerheads and powerfilters. These are far more efficient at moving water around. They can move tremendous amounts of water, sometimes even up to 100X tank volume/hour. Do you want that kind of turnover, in most situations you don't' so pick a powerhead small/large enough for your setup; or go with multiples in larger tanks.
Keep in mind that in most cases freshwater setups require less "turnover" than marine counterparts. There is more but I hope you get the point.
There are 3 cases that come to mind immediately regarding usage of airstones. You'd still use them in tanks that require very gentle water flow (some planted or fry tanks) and in case of an emergency in tandem with battery operated air-pumps. The 3rd is as above but with bait buckets
Happy fishkeeping.