Hi @Davidson
The when a file is created in OneDrive trigger on a flow triggers every time a new file is added. So if you are bulk adding files to onedrive, the upload time for all of these files might not be the same. 
Yes, you can send multiple files in a single email by creating an array of attachments. So in the flow, may be you can do as, when there are these bulk files adding, create a folder and put those files in that folder. List the folder and for each file in the folder, you can get the content of the files and create an array out of it and finally send the files as attachments. Note that the List files in a folder action is limited to return 20 items only.
Hope this Helps!
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