Understanding Spanning Relationships
Knowledge Article Number  000006372 

Description  Upon creating a new formula (in a formula field, field update or validation rule), you might get an error similar to the following: You have reached the maximum number of 10 object references on ObjectName
You must remove at least 1 relationship(s) in order to save this Formula Field.
Related objects referenced in this Formula Field:

Resolution  You receive this error because you have reached the limit of spanning relationships (also known as crossobject reference) per object.
Example  One spanning relationship
When creating a formula on Object1, you are allowed to reference another object, like this:
When creating a formula on Object1, you are allowed to reference another object, like this: The limit of spanning relationships per object is 10. This means, that an object, can only contain up to 10 different object references.
Please note selfreference lookup relationships are considered a "different" object when counting the number of spanning relationships. For example, if you have a lookup on the account object, looking up to itself and you use this relationship field in a formula, it will count as another spanning relationship. For example, assume you have the following scenario:
You create a formula field that looks like this: Then you create a validation rule that looks like this:
Understanding formula errors related to spanning relationships Example 1  Formula field contains more than 10 object references The error seen above, has a list of the relationship fields (in this case lookup fields) that are being referenced in the formula field that you are creating. Example 2  One formula field has 8 object references. A new formula being created contains 3 object references, hitting the limit of 10 references per object. The error above shows that 3 unique objects are being referenced on the new formula field however, another formula field (called test) already references 8 different objects. Example 3  Same as the above but one additional formula field already exists. This specific error can sometimes be misleading. The error message shows all the object references on this particular object, but it doesn't mean that all the fields being shown, count as an additional reference. For example, you will see that the field Test references Object9. Then, the formula field called Test2 also references Object9. This is considered as one spanning relationship. With this, it is important that you understand that if an error message shows the same object multiple times, this only constitutes a single spanning relationship. In the example above, the Object1 has 8 spanning relationships (despite the fact that the error shows 9 conflicting fields). In order to view the current use toward the limit of spanning relationships, we may either: 1) Create a test formula field using multiple crossobject references in an attempt to exceed the limit, and then expand the details of the error message, OR 2) Manually count the number of crossobject references within each formula field, workflow rule, and validation rule on the object For information about increasing the limit of spanning relationships per object, visit this article Increasing the Spanning Relationships Limit
