By default it should apply to all the document libraries in the web if the feature scope is Web, and to all the document libraries in the site if the feature scope is Site.
From your question I assume you have specified ListTemplateId="101" in your definition file.
Another possible case: if the event receiver throws an exception you will not get any message back and might feel like there are no event receivers.
Check the event receivers registered for a document library that you think is not firing the events.
Add-PSSnapin Microsoft.SharePoint.PowerShell –erroraction SilentlyContinue
$web = Get-SPWeb -Identity http://....
$list = $web.GetList($web.Url + "/lib name”)