Hi, Alejandro - First off, I would start with one lens and see how it goes, then add others if the need arises. In my opinion - for many reasons - the banquet formats are at their best with wider lenses.
Regarding coverage, ignore those who say that, IN THEORY, this or that lens won't cover. Get it from the horse's mouth. Having said that, I can state that the 14" f/7.7 Goerz Dagor will cover the format with some (but not a lot) of movement. (And for most landscapes you won't need a lot of movement - just a little rear tilt for DOF, which doesn't eat up coverage.) Sharp, high contrast - I recommend it as a good "wide" 12x20 lens. Patrick Alt - who repairs these big cameras for a living and knows what he's talking about - says the Nikon 450M covers the format, and he recommends that lens. (There was a piece in View Camera by him about this a couple of years ago.)
The critical specification you are looking for is Image Circle, and in 12x20 the magic number is 585mm as an absolute minimum. Realize that most manufacturers list the circle of full illumination (i.e. NO mechanical vignetting) at f/16 or f/22, therefore these numbers are very conservative. You will usually shoot 12x20 at f/64, and will find many lenses that will cover even they are not "supposed" to. Listen to those with practical experience, or get the seller to give you a return privilage if the lens doesn't work for your format.
Good luck!
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