Convert to grams per liter, and then use molecular weight of the compound to convert to moles per liter.
First you need to convert grams per ml to grams per l.
Normally, to convert ml to l you would divide by 1000 (since there are 1000 ml in a liter, ml is smaller than l).
However, the number of grams per liter will be higher than the number of grams per milliliter (since a litre is a larger volume), so to convert from grams per ml to grams per l you multiply by 1000.
Now to convert to grams per liter to moles per liter, you need to use the following equation:
moles (mol) = mass (g) / molecular weight (g per mol)
Divide your grams per liter value by the molecular weight of the compound to convert to moles per liter.
1.2 grams per milliliter NaCl
1200 grams per liter NaCl
Molecular weight NaCl = 58 grams per mol
1200 / 58 = 20.7 moles per liter