Reading this article, and specially this paragraph:
The SKU and Scale of the App Service plan determines the cost and not the number of apps hosted in it.
Can I create as many Web Apps as I need into the same resource group with the same App Service Plan?
For instance, I have a RG named S1-Resources and a S1 App Service Plan. If I create for instance 3 Web Apps into that RG linking them to the App Service, Will I be charged $0.10/hr for each Web App?
I found a good explanation here
Basically yes you can save money by hosting multiple apps in one app service plan - if you scale this plan to 2 instances, and you have 2 web sites running, they will both run on both instances.