Office 365 / Microsoft Graph service level agreement

We would like to use Excel (via Microsoft Graph and One Drive) to perform calculations in our application.

Potentially this would involve creating a workbook session for each user and performing tens of read and write operations per minute in each session.

Where can I get information about permitted number of concurrent sessions and requests per minute for an application, peak and average throughput limitations, and how we can scale this (e.g. purchasing higher service level etc.)?

Also, it would be great to speak to Microsoft support to directly discuss our requirements. Which is the appropriate channel to use for that?