Pls refer this link to understand the limits https://success.salesforce.com/ideaView?id=08730000000Lh1XAAS https://help.salesforce.com/articleView?id=reports_report_type_guidelines.htm&type=5 https://help.salesforce.com/articleView?id=reports_report_type_objects.htm&type=5 https://success.salesforce.com/ideaView?id=08730000000l6EsAAI
You can Add up to three child objects.
Pls refer this link to understand the limits
https://success.salesforce.com/ideaView?id=08730000000Lh1XAAS
https://help.salesforce.com/articleView?id=reports_report_type_guidelines.htm&type=5
https://help.salesforce.com/articleView?id=reports_report_type_objects.htm&type=5
https://success.salesforce.com/ideaView?id=08730000000l6EsAAI