From Knowledge Article
Each object has a limit of 10 spanning relationships.
When trying to create a formula that is referencing more than 10 objects you may receive the following error message:
"You have reached the maximum number of 10 object references on [Object name]
You must remove at least 1 relationship(s) in order to save this [Workflow Rule/formula field/..]."
Hitting the limit of 10 is a warning sign that the logic is getting too complex for the formula engine.
We allow the increase to a hard coded maximum of 15 so that you aren't blocked, but it's an indication it's time to start considering programmatic logic.
As possible workaround please verify if you can apply any of those best practices:
Possible other ways to expose data in formula fields. For example if you want to see information on an object 1-2 levels away on an object’s detail page but not report on it, you can require users to hover over the lookup link to view the details.
If you have multiple references to the same type of object (for example, you reference four different contacts in formula fields, each with a different purpose), consider creating a junction object so you can display the referenced objects in a related list.
If you have references that are only used in validation rules, rewrite those rules as an Apex trigger.
If any of the references are used by only one component, convert that to a trigger.
If you are spanning multiple levels of a hierarchy such as parent accounts, cases, or a hierarchy of custom objects, see if it’s possible to flatten the hierarchy.
To have Salesforce.com Support to increase the Maximum Number of Spanning Relationships Per Entity limit on your organization, please take the following steps:
1- Have a System Administrator to log a Case with Salesforce Support
2- Please mark as "feature activation" on the General Application Area
On the case description, please specify:
Feature requested: Maximum Number of Spanning Relationships Per Entity
Organization ID where you want the feature to be enabled (Navigate to Setup > Company Profile > Company Information):
I am the system admin in charge of this feature.
Limit requested (maximum possible 15):
3- Our Support Team will review the Case and action the request as needed.
Please vote the idea to increase the object references and here is the idea exchange link