You are attempting to attach too many files at once
When I try to add multiple attachments to my email I get the following message;
“You are attempting to attach too many files at once. Try reducing the number of files being attached.”
The total size of my attachments is well below the sending limit and I’m not aware of any limit to the amount of attachments that I may send out.
Why am I still getting this error upon attaching files?
You usually get this error when trying to attach more than 20 files at once from a network share. When you try to add them from your local hard disk, you won’t receive this error.
It’s a bit of a half-successful attempt to refrain people from attaching many files to a message within a corporate network (where it is more common to work with network drives).
Although the limit isn’t configurable, nor can it be turned off, there are many workaround for this nuance.
A "Yes, thank you, attach anyway" button might have been more user friendly.
Update: This issue no longer exists in Microsoft 365 and Outlook 2016 and later unless you are using the Classic Attach File interface.
Workaround 1: Send shortcuts to the files
If you are indeed in a network where the recipient can also access the network share, consider sending shortcuts to the files or the directory containing the files instead.
To quickly insert many shortcuts to files at once see; Adding hyperlinks to files in bulk
Workaround 2: Attach the files in batches
If at first you don’t succeed… try again! Once you’ve added your first 20 files, you can add another 20, and again and again…
Workaround 3: Zip the files before attaching
Instead of adding many attachment individually, another option/workaround would be to contain all the file files that you want to send in a single zip-file. Many 3rd party compression utilities have a "zip and email" functionality to do this in one go from within File Explorer.
Workaround 4: Attach the files via drag & drop
My favorite method of attaching files (regardless how many) is not to use the “Attach File” method within Outlook at all but drag & drop them from Explorer into my message.
Since you usually need to browse to the files anyway, I’d much rather do that in a proper Explorer rather than via the small and limited “Insert File” dialog. You can quickly open a File Explorer window via the keyboard shortcut 
+E
Workaround 5: Upload the files to a file sharing service
If you reconsidered and don’t want to send the files as attachments after all or when their combined size is larger than you allowed to send, then you can also use a file sharing service.
In a corporate network, you might have access to SharePoint, OneDrive for Business or Microsoft Teams for this. Ask your helpdesk if you do not know.
For personal usage, Dropbox, OneDrive and Google Drive are popular file sharing services. You can set permissions on the files so that only the intended recipient can access them.
