Most project never get to the point where they need more than 99.9% which is perfectly feasible with a single server. Interesting remark nonetheless.

Prod needs n + 2 if you want to survive losing one (machine, datacenter, whatever) while you have one out for maintenance. Buying three when you need one is pretty expensive; buying seven when you need five isn't as big a deal.

Staging just needs n > 1 to test that sharding works at all.