Hi Lucy,
Welcome to the AtoM community!
If you are talking about downloading, installing, and maintaining AtoM yourself, then the application has no predefined built-in limits. The number of records your AtoM installation is able to host successfully will generally depend on the resources you allocate to the server during installation. For most small to medium-sized institutions using AtoM in production as a public-facing catalog, we include some guideline recommendations for hardware resources here:
For very large sites expecting a lot of content, or a lot of digital object uploads, or a lot of traffic, you might want to increase some of these, and/or use other techniques to improve the responsiveness of the site, like a caching engine or a 2-site deployment. If these topics are of interest, let me know and I can point you to some more resources!
On the other hand, if you are asking about a hosted AtoM site given to you by a service provider, then any limitations will depend entirely on the service. For example, Artefactual offers a number of different hosting plans for those users who would like us to handle the installation, configuration, maintenance, and upgrading side of things. See:
With our most basic plan, we typically include 50GB of storage for digital objects, and we limit the ability to upload more than that. However, we also have options for you to pay for additional object storage if you want to stay on a Basic plan but increase your digital object storage. So.... it depends on the service itself!
In general, everything that Artefactual develops is released under open licenses, so you are free to study it, download it, install it, change it, and even sell your own services around it! Our documentation, slides, and other training materials are also released under Creative Commons
CC-BY-SA licenses, so again you can use, share, and modify them as you see fit. As a company, we try to make the outputs of our labor free, but the labor itself (our time, effort, and expertise) is what we charge for.
Cheers,