Well, it's not really the amount of airstones you use, it's how strong your airpump is.
When buying an airpump they normally tell you what it is rated for. Like 10 gallon, 20 gallon, ect.
Since you have a 45G tank, you'll want a airpump suited for around there.
Then after you buy the pump, you can choice any airstone you one to put in the tank, I strongly suggest nothing to big, something small and simple. I run a 5 dollar pump on my planted 5G tank at night. Airstone cost me about 0.45 cent. It's just a small cillender shape.
I suggest just getting an airpump suitable for your tank size, getting some airtubing (very cheap) and just get a simple airstone, nothing fancy since you run it only at night anyways. Not showing it off much with the lights off. My airstone was only 50 cents so you could prob get something like that.
Hope that helps.