Article 20(5) of the Tobacco Products Directive 2014 states that Nicotine containing E-Liquid must be in a bottle 10ml or less
If it is 0mg the bottle can be bigger.
1 x 18mg shot will make 50ml - 60ml of 3mg Eliquid
2 x 18mg shot will make 50ml - 70ml of 6mg Eliquid (if it fits in the bottle)
2 x 18mg shot will make 100ml - 120ml of 3mg Eliquid
4 x 18mg shot will make 100ml - 140ml of 6mg Eliquid (if it fits in the bottle)
Some short fills only leave enough room to increase to 3mg. If more is required you may need an extra bottle