Single boxes now are also insanely powerful compared to what we had back in the day too. Unless you are genuinely running the next Google, you might not even need to scale beyond one. As you said you might get a second box purely as a hot spare.

I remember we ran a pretty large UK website on 5 x Pentium II boxes, using a Perl stack which is not exactly memory efficient. Later we ran a chat server for hundreds of thousands of school kids on a single server which was (if I remember this right) a Pentium III with 128 MB (yes, megabytes) of memory and a spinning hard disk.