Chain vape at 12mg is definitely not advisable - its like chain drinking expresso.
Even without chain vaping, 12mg is too much for most vapers.
For beginners, an occasional overdose due to curiosity to try out as beginner is fine, as you can easily recover after few hours, most a day or 2.
For chain vaping esp. at subohms, you must always keep your nic low. Typically for most vapers, is 3mg to 6mg.
The higher the wattage, the lower your nic. Good thing is, vaping can all be tasted and felt, its not odourless and colourless like nerve agent VX gas, lol.
For long-term sustainable vaping, your body feel is the best guide.