The stadium is being designed for easy expansion. If they're selling out 21k every game for two or three years in the new stadium, they will have room to increase to 25k with relatively easy installation. I believe they are also working on ways to bump up to 30k beyond that with a second expansion if necessary.
We'll see how necessary that is, though. If the team is still mid-to-bottom on the table in year three at the new stadium, I doubt they'll have need to expand. But that's five years out, and attendance should be good until then just based on hype. Two years of 25k+ on average at Nippert behind the MLS hype, followed by a year or two of sellouts at the new joint regardless of how good or bad they are based on "new home" hype.
Year three in the new place will start to tell the story.