Hi @SpencerLarson,
So the ArcGIS datastore is a highly optimized SQL server, the services you publish to it is called Hosted Feature services, using it instead of a MS SQL server has a benefit in ram consumption and it is included in your purchase.
Then you can also have a look at PostgresSQL which is a free SQL server.
Then on MS Sql,
Express is free but it has a 10GIG Database limit 1410MB Ram limit and 4 cores and you will not be able to do a seamless upgrade should you decide to up the specs because of the way you access the express SQL is dns/sqlexpress versus just dns so if you change this you will have to re-publish the services.
Here in South Africa I mostly see a 4 core SQL deployment so that seems to be the standard.
On Portal,
A WebMap consists of services e.g Hosted Feature "Datastore", FeatureServer and MapServer "SQL" or "GDB", the WebMap itself is stored in ArcGIS portal but the services that is feeding it is coming from a data source, in order words you can have 100 WebMaps that have 1000 connections to a data source if they have 10 layers each.
I would start with a Standard SQL as this will give you room to upgrade if needed with minimum hassle.
and this is base on at least a Standard ArcGIS Enterprise purchase.
Hope it helps
Henry