This is one of the most common questions asked by new testers. For now I will take the cowardly approach and say, "it depends". I've read posts from people touting the number of tests they've written. This is a new metric as if "line count" wasn't already bad enough. There is a balance here that we are looking for. Just with the line count.

If your line count is small you're application probably doesn't do very much and if it is large then your application might be cumbersome and difficult to maintain. When you don't have many tests your code is more prone to errors, but if you've written too many tests maintainability disappears. But wait! Didn't I say earlier that tests make code more maintainable. Well, yes I did, but if you write too many it becomes difficult to change the code. If you test every single possible minute little thing in your application you're going to have a heck of a time changing anything.

When you're writing tests make sure you have just enough to give yourself confidence that your code is working.

I highly recommend against ever setting code coverage or test count goals. If you set goals for these you're just creating incentives to write more tests than is required. Too many tests can create the same problems as too much code. Why? Because tests are code.