Microsoft has released a guide to planning SharePoint Online on Office 365.
This guide will help you to size and give you a detailed architecture to archive your needs.
Use this guide before an implementation to design the project.
More information here – https://support.office.com/en-us/article/SharePoint-Online-Planning-Guide-for-Office-365-for-business-d5089cdf-3fd2-4230-acbd-20ecda2f9bb8?ui=en-US&rs=en-US&ad=US
