We are building an app that will be used by over 10k daily active users. The app has 100 screens with a lot of separate images (we do not use galleries). Each image control displays an image from onedrive as datasource.
My question: when a user opens the app navigates to a screen with 6 images on it does it count as 6 API calls?
Wow, 10k users is quite a large audience for your app. It sounds exciting to build an app for so many people. To answer your question the Microsoft blog says "When deploying your app to a large number of user, one has to keep in mind the connectors are throttled. PowerApps currently support 600 requests per minute per user and up to 30 concurrent calls per user." https://powerapps.microsoft.com/en-us/blog/performance-considerations-with-powerapps/
A large factor in the performance of your PowerApp will be your choice of datasource. The Common Data Service (CDS) or SQL are best suited to handle a high-load of transactions/concurrent users. Both of these options have a different licensing cost.
You may also want to consider using Azure Blob Storage instead of OneDrive. Azure has the best performance for displaying images in PowerApps. Like most things in the Azure Platform it is pay-per-usage.
Finally, an app having 100 screens gives me concern that you will go over the recommended number of controls in a single app. Per Microsoft: "Don’t add more than 500 controls to the same app. PowerApps generates an HTML DOM to render each control. The more controls you add, the more generation time PowerApps needs." In my humble opinion you should consider making multiple apps for ease of maintainability as well as to avoid breaching the controls limit. I like to build several apps with a narrow purpose rather than a single-mega app for everything
I know this post is quite lengthy but I hope you find it helpful
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Unfortunately, I am not sure about the number of API calls but if you look around in the newly minted Power Platform Admin Center you might be able to find a report to help test your OneDrive use case.