Let's use a rather common 50/50 sample recipe where you are using a Nicotine Base of 100mg/ml in PG and 20% flavoring.
To make a single 100 ml, because it is easier to see the percentages) you will use:
18 ml Nicotine Base
12 ml Additional PG
50 ml VG
20 ml Total Flavorings
Note that the 20 ml of flavor is counted as PG to make the 50/50 blend.
So if all that looks about right to you, for your first order you might want to get:
250 ml of 100mg/ml Nic Base
250 ml PG
500 ml VG
Also note that if desired you can get the nicotine in VG which can make the math slightly easier, but I assume you will using a DIY calculator.
BTW, I like to use 100mg/ml Nic Base because it gives you more versatility if you start to go into higher VG blends.
Any other questions, fire away!