Several thousand
Can't see how one would have made profit out of that. Was it just for kicks, RetiredCoder style? Throw away hundreds of thousands on a problem without caring much if it all gets lost or not...
Or maybe I don't know where to look to get my hands on several thousand RTX 4090 that cost something like 5 cents an hour.
An optimized CUDA kernel that targets some specific hash can do well above 7 Gkeys/s on a 4090, I guess that means only around 1000 GPUs would have been required.