A Catalog is a primary method of working in Capture One and it’s where all of the various steps in your workflow will be carried out, including image importing, capture (using a tethered camera), and image organization (viewing, sorting and grouping). A Catalog also allows you to apply all your image adjustments and then distribute your images.
The location of the original images (sometimes called source files) can be on any disk (local or external) and their location is referenced by the Catalog and recorded by the Library. As an option, the original image files can be stored physically inside the Catalog file. This is referred to as a managed workflow. When working with managed files, the images are always accessible. Capture One never alters your originals but if you store your images on an external disk and you do not have access to it when required, then Capture One can’t duplicate them to process or export them in further.
Whether you are using a managed or a referenced workflow or a combination of the two in the same Catalog, image files can be located and accessed using that Catalog’s Library tool. The Library tool in the Catalog tracks the imported original images in their folders or wherever they are located. The Library tool seamlessly keeps track of the corresponding variants and adjustments. After importing, you can use the Library to create projects, groups, and albums to further organize your images.
The number of images you can add to a catalog is virtually unlimited, but the catalog's performance is subject to the available system resources and read/write speeds and is less relevant if you have the catalog stored locally on a machine that is running the recommended system requirements.
Determining a specific recommendation for the number of images to add to a catalog is challenging due to various factors, such as hardware, image size and location, preview size, number of adjustments, and the number of collections/albums, among others.
As a general guideline, if you observe that your catalog is becoming slower, it's advisable to create a new catalog and continue importing your images there. Alternatively, you can export your catalog and split it.
NOTE: Capture One Express uses a single Catalog. There is no option to create a new Catalog when there is another Catalog already.
Learn more about Catalogs in this tutorial: