This settings obviously requires ALLOW_ATTACHMENTS to be true.

This does not restrict the total number of attachments that can be added to a campaign. It only restricts the amount of attachments that can be uploaded in one go. So the default 1 will be fine, because if you want to add more, you can add them one-by-one.

The total number of attachments is unlimited, although the SMTP server will probably have some maximum.