1 or 2 SVS ultra 13 subs for 20x24x9 dedicated HT room?

Depends on how loud you want your bass to be. Two subs gets you a 6 dB increase in volume. As far as the plugs, it depends on how deep you want to go...One plug gets you 15 Hz, two gets you to 10 Hz (with some loss of volume).

output wise, one is enough.

two will give you a better frequency reponse.

i'd plug one port each if you have two.

You need at least three.

If you like to feel the action in the movies, I recommend 2 PB13s with 1 port plug (15Hz mode). If you don't listen very loud (as in moderate volume), 1 PB13 with 1 port plug should be enough.

Unless you can get a good price break from SVS for buying two at once, I would try one and see if you need/want to add another.

i think there's a standard discount for pairs.

Overkill? What's overkill?

No one knows what is enough for you. You will only get a 6db increase if they are colocated. If they are in different locations you will get somthing closer to 3db. But, you can get a smoother response but it will require some time in dialing in the proper phase settings.

Only you will know, get one and try it, if you want more get two.

I have two 18"s and 4000 watts in a 1700 cubic foot room....

You can get the 5% discount if you buy two at once or get a second one later. With todays market, you may be able to negotiate a better price when you buy two at once.

right you are

Low frequencies below the x-over point can still sum to achieve a 6 dB output advantage because the 1/4 wavelength summation is still met with subwoofers positioned 3-4 meters apart.

Below 40 hz you should expect a maximum 6 dB advantage provided you have set the gain levels appropriately and the phase levels. However this advantage will not be linear up to the x-over point.

--Regards,

Any pics of your room? How loud do you like to listen? Have you previously owned a sub?

One sub will be much easier for you to configure. Two subs can either give you additional overhead (when co-located) or smoother response with a little bit of additional overhead (when placed opposite each other).

What placement were you thinking (hence the request for pics)?