Alpines website suggests .75-1.75 per sub. I will be building a box soon for one 12'' tuned to 32-33 hz. They give these dimensons (external) for each sub 18x13.5x16.5 that will leave you with 1.7 cubic ft after vent and sub displacement.
You can always use the box calculator on Reaudio.com. If you type in the dimensions it will build the box for you so to speak. You can make it how you want and change the design around until you get 1 you like.
Do you know how to figure out the port length? If you use re calculator it will tell you the length.
My box after displacement for one 12'' will be about 2.4 tuned to 33hz leaving me with 2.1 cubic ft internal.
