Each mold is different, and how each person pours is different, so this is just what I do, as a hobby pourer.
I use the 6 oz injector. I have a smaller one, but it doesn't hold enough plastic, and I've found that the 6 oz will fill two Do-It ES ripper, 5" senko, or 3" grub molds, before the plastic gets too cool to inject. Sometimes one cavity won't fill all the way.
I also know I can fill either three 7" senko cavities, or five 6" senko cavities, or six creature bait cavities. Those are home made POP molds.
I know there are folks here who have figured out how to heat their injectors so the plastic doesn't cool as quickly, but I haven't.
I have hand pour molds from Lurecraft, and I use them, too.
I would say the 6 oz injector is a good starter. If you eventually find you need a bigger one, you can always get another 6 oz and the brackets and blending block that is used to do laminates. There is no rule against filling both injectors with the same color, so you wind up with a 12 oz capacity, plus you can make some really cool laminates, if you want.