These are some limits found in the 2007 version of SharePoint, specifically Windows SharePoint Services 3.0.

 

Site object Guidelines for acceptable performance Notes Scope of impact when performance degrades
Site collection 50,000 per content database Total farm throughput degrades as the number of site collections increases. Farm
Site collection 150,000 per Web application This limit is theoretical, and is dependent largely upon:

  • Performance of the database server on which the configuration database resides.
  • Performance of the Web servers in the farm.
  • Network bandwidth between the Web servers and the database server.
 This is not a hard limit, and assumes a single database server. Your environment may not be able to host this many site collections per Web application. Distributing content databases across additional database servers can increase the effective limit of the number of site collections per Web application. You should perform testing to determine the actual effective limit in your environment. Farm
Web site 250,000 per site collection You can create a very large total number of Web sites by nesting the subsites. For example, 100 sites, each with 1000 subsites, is 100,000 Web sites. The maximum recommended number of sites and subsites is 125 sites with 2,000 subsites each, for a total of 250,000 sites. Site collection
Subsite 2,000 per Web site The interface for enumerating subsites of a given Web site does not perform well as the number of subsites surpasses 2,000. Site view
Document 5 million per library You can create very large document libraries by nesting folders, using standard views and site hierarchy. This value may vary depending on how documents and folders are organized, and by the type and size of documents stored. Library
Item 2,000 per view Testing indicates a reduction in performance beyond two thousand items. Using indexing on a flat folder view can improve performance. List view
Document file size 50MB (2GB max*) File save performance is proportional to the size of the file. The default maximum is 50 MB. This maximum is enforced by the system, but you can change it to any value up to 2 GB. Library, file save performance
List 2,000 per Web site Testing indicates a reduction in list view performance beyond two thousand entries. List view
Field type 256 per list This is not a hard limit, but you might experience list view performance degradation as the number of field types in a list increases. List view
Column 2,000 per document library

4,096 per list

 This is not a hard limit, but you might experience library and list view performance degradation as the number of columns in a document library or list increases. Library and list view
Web Part 50 per page This figure is an estimate based on simple Web Parts. The complexity of the Web Parts dictates how many Web Parts can be used on a page before performance is affected. Page
Security scope 1,000 per list The maximum number of unique security scopes set for a list should not exceed 1,000.

A scope is the security boundary for a securable object and any of its children that do not have a separate security boundary defined. A scope contains an Access Control List (ACL), but unlike NTFS ACLs, a scope can include security principals that are specific to Windows SharePoint Services. The members of an ACL for a scope can include Windows users, user accounts other than Windows users (such as forms-based accounts), Active Directory groups, or SharePoint groups.

 Farm

The following table lists the recommended guidelines for people objects.

 
People object Guidelines for acceptable performance Notes
Users in groups 2 million per Web site You can add millions of people to your Web site by using Microsoft Windows security groups to manage security instead of using individual users.
User profile 5 million per farm This number represents the number of profiles which can be imported from a directory service, such as Active Directory, into the people profile store.
Security principal 2,000 per Web site The size of the access control list is limited to a few thousand security principals (users and groups in the Web site).

The following table lists the recommended guidelines for search objects.

 
Search object Guidelines for acceptable performance Notes
Search index 1 per Search server  
Indexed document 10 million per search index 10 million documents per index server are supported, and one search index per index server. This means that the effective limit of documents per index server is 10 million.

The following table lists the recommended guidelines for logical architecture objects.

 
Logical architecture object Guidelines for acceptable performance Notes
Shared Services Provider (SSP) 3 per farm (20 per farm maximum)  
Zone 5* per farm The number of zones defined for a farm is hard coded to 5.
Internet Information Services (IIS) application pool 8 per Web server Maximum number is determined by hardware capabilities.
Site collection 50,000 per Web application  
Content database 100 per Web application  
Site collection 50,000 per database  

The following table lists the recommended guidelines for physical objects.

 
Physical object Guidelines for acceptable performance Notes
Index servers 1 per SSP*  
Application servers running Excel Calculation Services No limit  
Search servers No limit Because 100 content databases are supported for each search server, the number of search servers required per farm is based on the number of content databases in the farm. For example, if there are 500 content databases in your farm, you will need at least 5 search servers.
Web server/database server ratio 8* Web servers per database server The scale out factor is dependent upon the mix of operations.
Web server/Domain Controller ratio 3 Web servers per Domain Controller Depending on how much authentication traffic is generated, your environment may support a greater number of Web servers per domain controller.

 

See here for complete source: http://technet.microsoft.com/en-us/library/cc287790.aspx