As of now, none of WWE’s upcoming four shows at the Barclays Center have sold out, according to Dave Meltzer. (NXT Saturday, SummerSlam Sunday, Raw Monday, SmackDown Tuesday)
SummerSlam and Raw are close and may sell out, but NXT and SmackDown probably won’t, though they’re still drawing well. In the past, the Barclays Center seated about 13,500 for WWE events.
For the past two years at Barclays, NXT, SummerSlam, and Raw all sold out in advance. However, Meltzer points out that the primary reason is likely not a lack of interest, but due to WWE increasing ticket prices. As of now, the cheapest available remaining ticket (not resale) for SummerSlam is $500.