I was already mostly done with my research when the other answers posted (thanks!) and they suggest quantities that match mine, but I don't want to lose the extra information I gained, so here's the results of a survey of ice cream recipes.
I looked at the first page of results on Google searching for "vanilla ice cream" and recorded 3 non-custard recipes, and 6 custard recipes. I recorded the liquids, sugar, extract, salt, and egg yolks (in the case of custard) and found the following:
The range was fairly large, going from 1 cup sugar per 6 cups liquid on the low end to 1 cup sugar per 3 cups liquid on the high end. The average was right around 1 cup sugar to 4 1/4 cups liquid.
This was a little more variable. On the low end some recipes called for 1 teaspoon vanilla extract per 4 cups liquid, and on the high end some called for 1 teaspoon per cup of liquid. The average here was 1 teaspoon per 2 cups liquid.
Interestingly, unlike sugar, custards and non-custards differed a little in the amount of vanilla used, with custards using less. Non-custards ranged from 1 teaspoon per 1-4 cups of liquid, averaging to 1 teaspoon per 1.5 cups liquid. Custards ranged from 1 teaspoon vanilla extract for every 1 1/2 to 3 cups, averaging to 1 teaspoon per 2 1/4 cups.
When it was included, it ranged from 1/8 teaspoon per cup liquid on the high end to 1/8 teaspoon per 13 cups liquid. The average is 1/8 teaspoon per 3 1/4 cups liquid but 1/3 of the recipes didn't include it at all.
For the custards the egg yolk recommendations varied widely, from two egg yolks per cup of liquid down to one egg yolk per 3 cups liquid. It looks like there's a lot of opportunity for one to choose a ratio that suits their taste and texture desires. The average was one egg per 3/4 cup liquid.
For 9 cups of liquid (about 1/2 gallon prior to churning)
- Sugar: between 1 1/2 cups and 2 3/4 cups. Average is 2 1/8 cups.
- Vanilla: between 2 1/4 teaspoons and 3 tablespoons. Average is 1 1/2 tablespoons (4 3/4 teaspoons)
- Salt: if included, between a pinch (1/16 teaspoon) and 1 teaspoon. Average is 1/4 teaspoon.
- Egg Yolks (custard): between 3 and 18 eggs. 11 is average.