Most photographs are a unique artifact. This is especially true for older photos, before about 1920 or so. and it may be true for modern photographic processes also, such as 35mm slides and other processes. Historically, it was rare that multiple copies were made from the same negative; either glass or acetate. Unless the negative has survived, the original photo may be the sole surviving print. If the negatives survive, technically it is possible to make a photographic print from the original negatives, but especially for the older glass negatives, this may be very expensive and/or difficult to do. It much more efficient to make a digitized copy of the negative and then "develop" the copy using a program such as Adobe Photoshop or a similar program.
It is interesting, but the quality and stability of the very old glass negatives is usually better than later acetate negatives. Perhaps a little history would be helpful here.
Photography went through several major phases beginning in the earliest times from about 1800. The claims to the earliest photographs all fall in the mid- to early 1800s with the "Boulevard du Temple", a daguerreotype made by Louis Daguerre in 1838, the photograph generally accepted as the earliest one of people. See Wikipedia: History of Photography. The photographs made by the process developed by Louis Daguerre became known as a Daguerrotypes. Although many of these early Daguerrotypes survive, they are usually considered priceless museum pieces. Each Daguerrotype or other similar process photograph was an original and could not be easily duplicated.
In the second stage of photography, (of course this is greatly compressed summary) process were developed to create a "negative" image from which multiple "positive" images could be made. One of the most successful of these processes involved using glass as the medium for the negative image. Without going into a discussion of how this worked, suffice it to say that the glass negatives that resulted were often of very high quality that matches the quality of prints made today by any other process.
In the final stage of photography, negatives were made of acetate or other similar substances and of course, as digital photography took over in the last ten years or so, film photography has become a rather specialized "historic preservation" hobby.
I am frequently approached by people with questions about how to preserve their personal collections of photographs. I always emphasize the importance of preserving as much as possible of their photographic heritage. As I have already stated, it cannot be replaced. Once a photograph is destroyed or the negative destroyed, it is gone forever. I will come back to this subject in the future, but right now I will give you a basic link to a site that can get you started with the preservation of your photographs no matter how new or old they are:
Please start with the Library of Congress Preservation Section. The Library of Congress is very active in both artifact (photographs) preservation and digital preservation (the preservation of digital images).