I don't have a definitive answer for the blog, to be honest. I just add four more dynos (to bring it up to five total) and it seems to be fine. I figure it's only for a few hours, so I can be sloppy about it without needing to worry about huge costs. Wish I could be more scientific about it!

As for the number of dynos when Soundslice was on the Reddit homepage, I think was using 10 at that point, but, again, it was a totally non-scientific thing. 10 did the job, but fewer dynos might have been just as fine.

Thank you for responding. That's exactly what I was looking for, a ballpark figure for what can get you through a peak in traffic.