I've gotten as much as 60,000 in 24 hours if it stays on the front page. Recently I had one that hit 4000 in about 20 minutes but some circuit breaker hit (which I have no clue about, not really controversial or anything) and it dropped instantly to page 5 so the traffic died quickly. Generally at the top you might see as much as 10/s peak. I've seen 500+ concurrent but of course that is based on whatever Google considers concurrent.

Never had any trouble serving it (my own code, LAMP, on Amazon micro instance). programmer.reddit.com is a little lighter. Ancillary traffic (other sites) from a front page post might add 10% or so over time.