I started in 18 and it didn't hit the spot for me, then I went to 24 and that was fine, I came down from 24 to 18 again as soon as I started using better equipment.
In hindsight I wasn't vaping very well I was taking shorter sharper drags, sometimes lung inhales on my cig-a-like and as a result it was't getting much of the nicotine in the liquid into my system. I upgraded to a Lavatube and a vivi nova and immediately started getting nic headaches.
So if you're using a cig-a-like then definitely get higher strength liquids it might be 24mg/ml liquid but you're not really getting all of that 24mg/ml to start with.
also as other people have said your body will let you know when you've had enough. If you start getting a headache - that's one of the first signs that you've taken in too much nicotine either switch to a lower strength juice or vape a little less often.
Another thing is if you've just switched totally from smoking to vaping then the way the nicotine is delivered to your body is different and it can take a few days for your brain to adjust and the new habit to feel "normal" to you.
One good trick is to have a clearo/tank with the high strength e-liquid in and one with your normal vape in. If you feel you need more of a hit have a vape on the high strength stuff for a while then switch to the lower stuff again a little later.