You left some important information out of your question so I don’t have enough to give you a conclusive answer. You can start from checking a couple of my articles concerning combustion air supply for fuel burning appliances:
The amount of combustion air required for any fuel burning appliances depends on the BTU’s of the appliance (those should be listed on the label attached to your WH). The size of the openings providing combustion air depends on where that air is coming from (exterior, attic, crawlspace, another room, etc.). Since you didn’t provide any of that information I can’t really tell you if 2 x 6” diameter pipes will be sufficient.
House interior scenarios and corresponding vent sizes are explained inside the articles linked above (don’t worry about “Chicago condo…” title in the second article, it applies everywhere).
Since there are a few possible scenarios just click the “comment” button below this answer and; provide BTU’s of your WH along with a detailed description of its location (closet), what’s above, below and behind the walls, what area does this laundry room opens to?