SharePoint Limitations

When examining the SharePoint limits, you’ll see references to several terms that are specific to SharePoint.

In SharePoint, a web application refers to an Internet Information Server site that has been extended with SharePoint.

Each web application may contain one or more site collections. A site collection is typically a complete intranet, extranet, or Internet implementation.

A site collection is made up of one or more sites. A site is a dedicated section of the site collection for team collaboration, business intelligence reporting, records management, or the like.

Sites contain lists and documents and may have subsites underneath them.

The 2,000 rule: because stored procedure calls to SQL Server slow down as you reach 2,000 items, have less than that in a view on a List. Less than 200 ideally to have optimum performance.

Limits:
Site Collections in an Application:
50,000

Sites in a Site Collection:
250,000

Subsites in a Site:
2,000

Lists on a Site:
2,000

Items in a List:
10,000,000

Documents in a Library:
2,000,000

Documents in a Folder:
2,000

Maximum Document File Size:
2GB

Documents in an Index:
50,000,000

Search Scopes:
1,000

User Profiles:
5,000,000

Have less than 2000 AD users or groups in a SharePoint Group for the same reason. An AD group in a SharePoint group counts as one item, but the first time a user edits content in a site collection, they are added to the user list for that site collection. The best way to avoid this problem is to have lots of site collections so the users and groups are more spread out.

The bottom line is you can have as many readers as you like on SharePoint site collections as you like, but if you get to between 1,500 and 2,000 contributors (to reach that 64k limit) you will get errors.

More detail here: http://weblogs.asp.net/erobillard/archive/2008/09/11/sharepoint-security-hard-limits-and-recommended-practices.aspx