I have put in quite a few of these things. Be prepared to spend a a lot of money if you are in a 6k sq ft house. I couldn't tell you if you need 2 or 3, but a certain NFL player built a house for his mother in the area and it took 5 of the bastards. It really just depends on how many people are going to be using hot water at any given time. If you are alone in the house...or with just a wife...one would be fine. If you have several children or expect guests often I would do at least 2...3 to be absolutely safe.
Another thing is this - if you go with the electrical models, you must have at least a 60 amp service. I have never put more than one in a house at a time, but it wouldn't surprise me at all if your home's regular service would need to be upgraded. If you choose to go with gas...then most likely all of the gas piping in your home would need to be re-sized all the way from the meter. Honestly, I would say you are looking at a minimum of 4,000 to get 2 of these installed. It could skyrocket if certain things are not going to work out.
Another misconception about these things is that people think they allow you to get "instant" hot water. This isn't true...only a recirculating system will allow you to do that. If you have dreams of granduer about saving the environment and getting all your money back in energy savings by using these things - go ahead and forget it now. I am a plumbing contractor and I don't, nor would I currently ever consider, putting one of these in my home. They simply aren't that much more efficient than a decent regular old water heater - that salespitch you hear is just a bunch of beaurocratic bullshit.