The only thing which matters for your project

I think it’s a good test coverage.

I saw quite some projects: big and small, old and new, with and without test coverage.

And so far I’m quite sure that it’s possible to change a lot (no clear definition here) of code, if you’ve got a good test coverage.

How good the test coverage suppose to be?

Well, just enought to make it possible to change a lot in your project.

Some people say that it’s important to have 100% unit test coverage, some people unit test only business logic, someone prefer integrational tests without unit tests at all.

So far, as a minimum effort, I would suggest to do (A)TDD which will make all the critical areas covered by tests.

As a maximum effort — test coverage should be (100% x number of test types), so if you have integrational and unit test, test coverage should be 200%. And this number actually can be checked in you pipeline, if you like to be strict.