Hi
@MarieT and welcome to the planet.
to add to whats already been said. sadly you wont be able to get to 12mg strength by using nic shots. the only way to do this is to soure some 72% nicotine which is illegal to buy in the uk now to the public (but its out there) wink wink.
for 12mg i would recommend buying 10ml tpd tested bottles which already have nicotine mixed in them and are available in 3/6/9/12/18mg
nic salts are much smoother but go upto 20mg.
have a look at some of the 10ml suppliers here
POTV Vendors & Recommended E Liquid Suppliers
to name a few
88 vape
drip drop
LiQuid
vampire vapes