The nicotine stated is per ml so that is 18mg per ml
Don't bother trying to convert it to how many cigarettes that is because vapor is absorbed differently. possibly less immediately but it seems to hang around longer to me so you end up with a good amount. Judge by how you feel. You want a TH you like and not so much nicotine that you get over-nicced symptoms like headache, rapid or irregular heartbeat, cold fingertips, lightheaded or yucky feeling.
I started on 18mg then after 3 or 4 months I started to work my way down, lowering my dependence on nicotine and TH. I like 10-12mg a lot too. I am now all the way down at 4mg which is pretty minimal TH but at least I can vape a lot at times now and rarely over-nic myself.