Limit the number of files/ total sizes can be downloaded by user per hour in Sharepoint Online?

Duy_Nguyen_Duc 
This is going to be a challenge.
The most robust solution is to ensure all documents are encrypted when they are downloaded. This requires a serious amount of work and appropriate licenses.

You may want to look at the capabilities of Conditional Access to impose "download" limits. Again, the use of Conditional Access has license constraints and requires effort to implement.

Does your company allow use of OneDrive for Business client?
If so, that makes things even harder. The "user" can then simply accumulate all documents on a local machine using OneDrive client. If the documents are not encrypted they can be simply opened, copied, ...

Summary: blocking all options is going to be hard.
Paul