I think, as fans, we tend to under-estimate the number of empty seats when assessing the crowd.
And because I think this, I tend to adjust down from what I'd guess at. So I'm saying 22-23k.
In fact, reflecting on this again, I reckon I'm possibly still over-estimating there.
20k, for example, would mean approximately 1 seat in 3 across the stadium was empty, and I could well believe that was the case.
I'm certainly highly confident that it was a lot more than the 1-in-6 empty that 25k bums-on-seats would represent.