You can access the Object Limits page when planning how to customize a particular object, or to monitor the current usage and limits, such as the number of custom fields or rules applied.
To access the standard Object Limits page, navigate to Setup | Customize. Click on the name of the desired standard object, and then click on the limits, as shown in the following screenshot (for the Account
object):
Here, you can see usage details for the following: Custom Fields, Rollup Summary Fields, Custom Relationship Fields, Active Workflow Rules, Total Workflow Rules, Approval Processes, Active Lookup Filters, Active Validation Rules, VLOOKUP Functions, Sharing Rules (Both Owner-and Criteria-based), and Sharing Rules (Criteria-based Only).