I’d start by tuning PostgreSQL for 16GB of available memory and provisioning 3 unicorns per core (that’s core, not hyperthread – so a total of 12). On a 32GB box, that should leave you with about 8GB of RAM for Redis, disk cache, overhead, and “whoops that unicorn got a bit bigger than I was expecting”. Having 32-64GB of swap on hand, also, should go without saying.
Then, see what happens: if you seem to be bottlenecked on unicorns (and you’re not CPU bound), throw in a couple more. If your PostgreSQL queries start to chug, retune PostgreSQL to use a bit more RAM. At 39€/month, though, I’d very quickly consider splitting DB and web if that one box started to feel overloaded. It’s almost too cheap not to.