You're right that the existence and quantity of news does not necessarily indicate bias, however, bias is found not just in news that is reported but also in what news is reported versus what news is not reported. I personally find this to be where most of the egregious media bias is. For instance, if the media chooses to not cover left-wing initiated violence at a protest, but heavily cover right-wing initiated violence at a protest, that is still bias, even if the reporting about the right-wing violence is accurate.

It's a much more insidious bias too because its harder to point out. You will have to use non-mainstream sources as they are the only ones doing the covering, and the fact that they are not mainstream will be used to hand waive away the complaints.