As Aurora said, with your nic being suspended in PG and most flavors being PG based as well, you will have some PG left in your gallon if you want to remain at 50/50. So it really depends how much flavoring you use, some of my recipes get up to 30% flavoring and that has to be counteracted by VG. You will definitely be able to make a lot of juice though.
I would pick up some extra VG, at least 1,514.164mL. That's assuming 20% flavoring as a median.
You are going to need a lot of flavoring for that amount of juice. I might recommend two places:
1) Get a bulk retail account with TFA so you can get discounted bulk pricing on their flavors
Perfumers Apprentice - The Flavor Apprentice - World of Flavors
2) Bull City Vapor has some of the best bulk proving I have ever seen.
Bull City Vapor - Best Prices on DIY e-liquid Supplies
However, if you buy all of my 120mL Cap's V1 Vanilla Custard I will... be thoroughly disappointed! Only kidding lol. Not really. :/
Vape Happy,
Cullin Kin