From what I've read it depends on the desired rate of charge and not a lot more than that. About a year ago I got a new phone and it was clear it needed a minimum of a 2 amp to charge as designed. I have 2 A/C "wall warts" I can use with it. One takes in .5 amp and outputs 5v at 3 amps and it charges the phone fairly fast. It also provides 9v at 1.6 amps and 12v at 1.2 amps. The other outputs 5v at 2 amps and it charges slightly more slowly and will be warm in use. My wife's phone wants at least 2.4 amps to charge and will charge using 2 amps but again it will charge more slowly. Now for what it may or may not be worth, I have another (older) A/C plug-in charger which takes in .15 amp and outputs 5v at 1 amp. It will charge the more amp-hungry phones plus iPads and any other similar device. It just takes several hours to do it. So I wondered about the 9v and 12v output stuff and did some looking. Here is an article I saw back then which I thought was interesting:
https://www.pcmag.com/how-to/what-is-fast-charging
