Hi Patrick,
Honestly, it kinda of depends on your use case. If you're deploying a DEV instance, expect a minimal number of users, or just want to explore the functionality of ArcGIS Enterprise, the all-in-one ArcGIS Enterprise installer might be your best bet. It'll deploy ArcGIS Server, Portal for ArcGIS, the ArcGIS Data Store, and two ArcGIS Web Adaptors on a single machine.
Portal itself is relatively lightweight - really, it's just a website. ArcGIS Server is where the weight of the processing happens. I'd consider hosting the data store on a separate machine - especially if you're going to work with the Spatiotemporal Data Store (It's recommended that the SpatioTemporal Store be on it's own machine).
Here's a few links to help understand various deployment scenarios. Basically, you can scale as small as one machine, or scale out as required.
Deployment scenarios—ArcGIS Server (Windows) Installation Guide (10.5) | ArcGIS Enterprise
Deployment scenarios for a highly available ArcGIS Enterprise—Portal for ArcGIS (10.6) | ArcGIS Ente...
All this means is that without context, site and server sizing can be a challenging question to address, and it honestly depends on the use case, expected load, performance and availability requirements.
