Another TechNet capacity planning guidelines for SharePoint, a confirmation that you can store up to 1 million documents in a single document library.
Plan for software boundaries (Office SharePoint Server)
The most useful bit I found in this article is on scaling a document library as people often ask how many documents a SharePoint document library can hold before performance starts to degrade.
Throughput differences between flat document library vs. document library with folders
Throughput for certain operations decreases as the number of items in a folder increases.
The following figure shows the difference in throughput between viewing all items in a document library with and without the effective use of folders, which is critical for scaling. As shown in the graph below, throughput performance degrades as the number of documents increases when flat library storage is used. The quickest drop in throughput occurs when the total number of documents is less than 2,000, from 151 RPS (at 200 documents) to 63 RPS (at 2,000 documents). At 4,000 documents, throughput decreases to about 13 RPS, or an overall throughput decrease of over 90% from an empty library.
The following figure shows the relative performance between folder views when folders are used to store and organize documents, and an indexed view of a flat library structure. Each folder contains 500 documents created by different users. In this scenario, there is no significant throughput degradation up to 1 million documents for either scenario, provided that the number of items in the view does not exceed the performance threshold for your system. However, performance is better when folders are used.
As the number of items in a folder increases, folder view performance will gradually degrade. Note that the above results are estimates based on our testing, and results may vary in your environment.
Read more here:Plan for software boundaries (Office SharePoint Server)