I recently began using Google App Maker and in order to reference multiple tables from an external MySql Database, do I have to create multiple Data Models for each table or do I have to create multiple datasources inside one Data Model and adding all the fields in it? The database has 15 tables in total.
You need to create model per table.
You can connect your existing database to App Maker (in application settings) and import tables as models (create model -> existing model). So you don't need to re-create fields.
Please be aware that as you change models in App Maker the database from application settings changes accordingly. So I would recommend use prototype database in application settings and production one in production deployment.