Background: While manually testing an application in scenarios, where an application has complex functionality features, supporting multiple platforms(Web/mobile/Web APIs)/versions(OS Versions) where most of the features can interact with other features in multiple ways which give rise to a significant number of scenarios due to permutations.
Problem statement: After validating the happy paths as per the requirements, where functionality can be further tested in thousands of scenarios if each and every combination of features is tested and if we keep testing more and more with 'what if' scenarios and keep finding more issues; then as a tester, how one gets to know/decide when to stop testing?