By default, Microsoft CRM 2011 integration with Sharepoint 2010 creates a document library for each entity, and a folder for each entity record in which you can add documents.
Ie: you have a folder for Opportunity A and another for Opportunity B.
Now, the rough recommendations for sharepoint sizing are as such :
Sharepoint 2010 : 5000 folders and 5000 documents per folder (per document library)
With the CRM Sharepoint default integration, the number of documents per folder is not really an issue but the number of folders can.
Note that CRM 2011 only creates a folder when needed (ie: when documents needs to be uploaded), so this reduces the impact somehow as you can have 10.000 customers with only 20% requiring document management.
But what if you have something like 30K customers with most of them needing documents attached ?
Here's some options :
1) There's no real hard limits, so you can push more than 5000 folders in a document library but depending on your sharepoint infrastructure, the performance may start to degrade sharply when adding new documents or folder (as the 5000 limit is mainly due to table locking vs row locking on the database by the sql server)
2) Add more documents library. You could decide to have something like a document library by geographic region or another segmentation (large customer base are usually segmented for business reasons).
Then in the record itself,in the document part, you can add another location pointing to the new document library.
Usually, in a normal implementation, you would default the new location to the user instead of having it done manually as the CRM 2011 SDK offers the methods to manage Sharepoint Site and Locations.
3) Manage the Sharepoint integration without the default integration, which is definitely the option if you use Sharepoint 2007 as the folder management is only available for Sharepoint 2010.
You would then use the Sharepoint SDK to manage the integration (ie: vs option 2 which is using the CRM SDK)
For more informations on sharepoint sizing and limit, please read this document: SharePoint Server 2010 capacity management: Software boundaries and limits