You can validate the number of or even the type of attachments attached to a Nintex form using a new setting in the Attachment field itself. If you need to only require attachments some of the time see the conditionally require attachments tutorial.
Require attachments on Nintex Forms
1. Double-click on the attachment field on the Form:
2. On the Attachment Control Settings, expand the Validation section
3. You can set the minimum number of attachments, maximum number, or specify what file formats are allowed to be attached to the form.
Finished Example: the form gives me an error if I don’t have at least 1 attachment. The form also allows doc or docx file extensions files to be attached, but if I try to attach a file with a different file extension a popup error box appears: