The simple answer is it depends. How fast do you want the activities to process. There should should be ~8gb of memory or thereabouts (more if running multiple applications throughout the day). The SDK/API application is largely CPU dependent though. Throwing more CPU’s at it in our experience does not move the needle a great deal. The faster the CPU, the faster the applications can get into the code, execute the process, close the connection and then loop through the sequencing.
An example from an account which has used our OIS application for years: They were on a dual core, 1.8Ghz processor server for a long time. They upgraded into our AWS Private Cloud for Everest and r5n.large instance which has a sustained dual core, 3.1Ghz processor. The processing speed for OIS on a typical batch of 500 orders importing was cut in half. Instead of taking 2.25 hours for processing, the imports took just over an hour.