This got me thinking. Take a look at this article from Football Ground Guide which was published in January:
We are 26 games into the EFL Championship season, so lets take a look at the average away attendances this season to see which club has the biggest away following.
footballgroundguide.com
On that basis, we would probably expect 2,000 sellouts from 15 clubs: Sunderland, Leeds, Sheffield Wednesday, Leicester, Ipswich, Coventry, Birmingham, Middlesbrough, West Brom, Stoke, Preston, Norwich, Hull, Huddersfield and Rotherham.
Plymouth, Southampton and Blackburn would be over 1,000 but well short of the capacity unless they are going well.
That leaves only Bristol City, QPR, Watford, Cardiff, Millwall and Swansea below the 1,000 mark, and they would still be decent numbers.