A typical PHP application that does a bit of database updating per request, gets some new data from the DB and templates it should handle 20 requests per second on a single $20/month VM. And in my experience from the last years, VMs have uptime >99.99% these days.
What made you settle on a multi-machine setup instead? Was it to reach higher uptime or were you processing very heavy computations per request?
What made you settle on a multi-machine setup instead? Was it to reach higher uptime or were you processing very heavy computations per request?