Last Friday, the 22nd of May, the Strange Horizons fiction department received our 25,000th valid submission.
I don't know which one it was, exactly; there's enough inaccuracy (due to occasional invalid subs making it into the database, among other things) that I can't tell precisely which one was the 25,000th. But I'm confident that we're within a hundred submissions on either side of that number.
Approximately 11,000 authors have submitted to us, so they've submitted an average of about two and a quarter stories apiece, though there's so much variation in that that the number is probably largely meaningless. (Most authors only ever send us one story, while a few have sent us over 40.)
Our first submission arrived in late June of 2000, so that's an average of a little more than 8 and a half stories a day (counting only the days when we've been open to subs) since then.
It's been about 15 months since we hit 20,000, but we were closed to subs for 4 months of that period (last year's summer and winter closures). So we've been averaging about 15 subs a day (counting only days we were open) since that 20,000 milestone. All of which suggests that we'll probably hit 30k sometime in the middle of next year, depending on how long we close for at the end of this year.