You are attempting to edit / add cross-object formula fields in your Org and encounter the following error: You have reached the maximum number of 15 object references on 'Object Name'

This error is due to a Salesforce limitation of 15 cross-object formula fields per object.

Please review the following Salesforce Knowledge Articles for additional detail:

https://help.salesforce.com/articleView?id=000171213&type=1&language=en_US
https://help.salesforce.com/articleView?id=Understanding-Spanning-Relationships&type=1&language=en_US

Upon request, Salesforce Support may be able to increase this limit from 15 to 20 for you, however 20 is the hard maximum limit.