Around 4,000.
As I said above - a lot of expense for just 3-4,000 seats and not strictly financially sustainable. With all the jiggery pokery, new roof, cantilevers, escapes, structural strengthening, house clearances and roads (but for me, i'd leave the road where it is and cantilever over it). It might even be cheaper to knock it down and start again.
There are better and probably more cost effective ways of making things better. Like safe standing at 1.8:1 (but that's not simple either). Of course, making it bigger and doing 1.8:1 - now you're cooking mit der gazoline!
In that mad context, the limiting factor is the viewing distance, which gets you about a 17k Kop sitting down. Of course 30k standing up really is just mad...of course, it is. Isn't it?