My guess is they would close the upper deck unless they were getting exceptional attendance.
There's reason to think they could do ~12,000.
League averages this year
Saskatchewan: 13,459 (capacity: 15,195)
Buffalo: 13,046 (capacity: 19,070)
Colorado: 12,815 (capacity: 17,809)
Calgary: 12,594 (capacity: 19,289)
Philadelphia: 10,805 (capacity: 19,306)
Toronto: 9,477 (capacity: 18,800)
San Diego: 7,769 (capacity: 12,920)
Vancouver: 6,833 (capacity: 18,910)
Georgia: 6,699 (capacity: 10,500)
Rochester: 6,440 (capacity: 10,424)
New England: 5,527 (capacity: 7,074)
Although the league wide average attendance went up, most teams (most notably Buffalo, Saskatchewan, Colorado, and Toronto) were actually down due in part to a labor stoppage that resulted in multiple early season games being cancelled and then rescheduled later. The reason the leagu-ewide average went up is the expansion Philadelphia Wings as well as the Vancouver franchise doubling its attendance (their number looks low now, but they just had their first year under Canucks ownership and were previously struggling to draw 3,000 in a tiny arena in the suburbs).
Next year's expansion teams are Halifax (capacity 10,595) and New York (Nassau; capacity 13,917)
There have been many years where Colorado or Buffalo averaged 15,000-18,000. One season the Colorado Mammoth even out drew the Avalanche and Nuggets on a per-game basis (obviously they play fewer games at lower prices, but it was still impressive).