This question just popped in some kibitz in a rengo among two 9d-8d teams. I guess this can be precisely answered with access to the server logs, so... I would bet for some tartrate game.

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I wasn't around then, but I think Guo Juan's first audio lecture had 1500 or so. There was a Willzhang slow game I saw that was 900+.

The server doesn't log who watched what game. Unless you comment and it gets into the SGF log, there is no way to tell. So this question can't be answered exactly.