They use multiple servers for redundancy, but they are using only 5-10% capacity per [1], so they say they could run on a single server given these numbers. Seems like they've since moved to the cloud though [2].

[1] https://www.datacenterdynamics.com/en/news/stack-overflow-st...

[2] https://stackoverflow.blog/2025/08/28/moving-the-public-stac...