In the Data Loss Prevention Enforce manager, Is there a maximum number of custom attributes I can define?

You can only have 50 active custom attributes at a time.

You can delete attibutes and add attributes, but the total that can be use is 50.

If you try to add a 51st attribute, when you try to save the attribute you will get a red banner with the message,

Maximum attribute count is reached.  No more attributes can be added.