How many documents can we store in Appian?

What is the purpose and approximate total bulk filesize of the "many millions" of documents you anticipate storing?

I'm not sure whether there's a hard limit within Appian, though obviously environment sizing would affect both performance under a heavy load of documents as well as the total size you plan to store. Nominally Appian isn't supposed to be an alternative to bulk cold data storage services, though if you keep an eye on optimization it might be ok.

My one suggestion would be, where possible, to create ZIP packages of older collections of documents, or really any where the intent is to have them for archival purposes as opposed to active ongoing use. This would allow the filesystem to be unburdened of quite as many files it needs to index, as well as potentially allowing for some easy data compression.

There is no limit to the number of documents that can be stored in Appian. Other than making sure you have enough disk space and memory, there isn't anything special you need to do in terms of configuration. In terms of performance, you will want to be mindful in how you design your applications to ensure that the design is appropriate for your intended usage. For example, if you have 100 million documents, and then you give users access to a field that searches all 100 million documents... that search is, of course, going to be slower than it would be on a site that only had 10,000 documents. On a similar note, it's important to remember that documents are design objects (just like process models, interfaces, etc), so if you try to search through all of your design objects (for example, from the objects tab in Appian Designer), you would want to exclude documents from that search.

There are installations running in production today that are taking advantage of Appian's document management capabilities to handle millions of documents or more. As long as you keep some basic considerations in mind, storing and managing large numbers of documents shouldn't be an issue.