Double IPA

I want to do a double IPA brewed exclusively with columbus hops and i want around 8% ABV. How much yeast should I use? ALSO how much hops/malt extract/malt is too much for a 5 gallon batch?

Check this site out for info on how much yeast to use. As far as how much ingredients are too much.... you're probably not gonna get there with a IIPA.
I'd recommend using some software (there's free stuff online) to design your recipe. Very useful.
FWIW, I have a big (1.075), all columbus IPA in primary right now (ok, I used horizon to bitter, but all columbus late). Are you planning your's as an all extract, or a mini mash brew? Maybe I can help ya come up with a basic recipe idea if you're interested.

Tough to brew a good Double IPA via extract and spec. grains... especially if you're topping off with plain water at the end of the boil.

I suggest brewing this via partial mash and doing a full volume boil. Are you open to this?

Give us more specific goals, i.e. what you want/expect from this beer and we can help you out.