Welcome Jasmin, You do not really look new here, but for people who are....
Nicotine strength knows many factors. And also even with that, there is not a fixed right strength.
First of all, Nicotine will hit you harder when you vape on higher power, like big direct lung clouds, my limit there is 2-3mg, sometimes even lower. With my all day tootle vape device (mouth to lung) I prefer 6mg, but i started out at 18mg, i just slowly lowered it to 6 and that is fine. I do also sometimes like to have a few 20mg drags from a disposable, i just feel the effects harder.
There are also two effects. The throat hit and the nicotine hit to your body itself. Both are important.
There are two types of nicotine, The freebase one and the nicsalts, the first one is harsher and is commonly vaped at lower mg, nicsalt is smoother and can be vaped with higher strength with milder throat hit, but you will feel it faster.
So the right amount is where you can enjoy your vape, best throat hit with the best nicotine effect. when the amount is too high, you will just vape a bit less.
In my opinion it is best to start a little bit high, to keep the urge for a real cigarette away.
Another very important thing is, when you stopped smoking and start vaping, there is a chemical from the smoking that will stay for a couple of weeks in your body, this chemical will make nicotine hit harder. This means that after a couple of weeks, you will feel the urge to get a cigarette. This is because that chemical left your body and nicotine isn't hitting as hard. Then it is smart to go a little bit higher in nicotine strength.
After a while you can then slowly lower to an amount where vaping feels most comfortable. The only reason to go lower is if you completely want to quit. you can than slowly lower it to 0mg. But don't go too fast.
* In short, you need to try it and tweak it.