There was an excellent “plain English” guide to archival management in small historical organizations published some years ago by the Pennsylvania Historical and Museum Commission. I believe that the author was David Carmichael.
Bear in mind, however, that not all two-dimensional items (documents, photographs, etc.) are truly “archival” in the sense of being substantial bodies of material deliberately preserved and arranged to address the purposes of some organizational entity.
If what you have are more or less random individual items acquired from multiple sources along with other collection objects, you may find that you’ll be needlessly tying yourself in knots by attempting to implement a separate organization and numbering system merely because they’re two-dimensional, rather than treating these like other objects in your collection.