In Task boards of our TFS 2017 (Microsoft Visual Studio Team Foundation Server Version 16.131.28601.4) I see the message that I can not add more than 10 style rules and I dont know where this settings has come from :
This is not a user configured setting, it's a system limitation. Same on Azure DevOps 2019 latest version and Azure DevOps Service.
When you added 9 style rules, you will see this promote message: you can add up to 10 style rules tfs
When you added 10 style rules, you will not able to add any new style rules, the "+" button will also turn gray.
If you wan to add a new style rule. You have to delete a redundant style rule, choose the actions icon and select Delete. There is not any configuration to expand this limitation.