Back to your initial questions...
How much flavor for a liter of juice? I menthol only.
Flavor percentages aren't something most (all?) manufacturers divulge, otherwise they would be giving away their recipe and therefore they'd be out of business. No restaurant is going to give out the recipe for their dishes, same for eliquid manufacturers. Trade secret.
I'm thinking of buying a bit of bulk juice for stock.
If a juice is listed as 3mg nicotine regardless of size of container, what size container are we talking about?
Whatever size you buy. Container size has no bearing on the strength of the nicotine. 3mg is 3mg is 3mg. That is what you'll get, 3mg (per ml) strength.
A 1 liter (aka 1,000ml) bottle of 3mg (aka 3mg/ml) nic base has 3,000mg of nic in the whole bottle. 1000*3=3000
A 100ml bottle of 3mg (aka 3mg/ml) nic base has 300mg of nic in the whole bottle.
A 50ml bottle of 3mg (aka 3mg/ml) nic base has 150mg of nic in the whole bottle.
A 30ml bottle of 3mg (aka 3mg/ml) nic base has 90mg of nic in the whole bottle.
A 15ml bottle of 3mg (aka 3mg/ml) nic base has 45mg of nic in the whole bottle.
A 10ml bottle of 3mg (aka 3mg/ml) nic base has 30mg of nic in the whole bottle.
A 5ml bottle of 3mg (aka 3mg/ml) nic base has 15mg of nic in the whole bottle.
Therefore, if you vape 12ml of 3mg (aka 3mg/ml) strength eliquid per day, you consume 36mg of nicotine per day.
I’m looking at Liquid Nicotine Wholesalers out of Phoenix. My normal store bought juice 79/21 and 0.2% (2mg/ml) nic level. As I said menthol is my only flavor. I guess I answered my question about mg/ml. My only remaining question would be adding the menthol flavor
That's what DIY eliquid calculators are for...
DIY E-liquid | Steam Engine | free vaping calculators
There are many others, both online and client-side programs/apps.
What DIY calcs can't help you with is how much flavor you need to get the perfect taste. That's something for you to figure out. Taste is subjective and there are many other factors (like wattage you vape at, etc) that determine what it will taste like.