You can access Object Limits pages 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 Limits, as shown in the following screenshot (for the Account object):
Here, you can see usage details for these: 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).