I think it is supposed to be all of them. But who knows. I asked for a way to know which notifications have been read but it hasn't happened. I'm still frustrated by notifications. I click mark all read often knowing I haven't seen them all. The number by the bell might be 8 but who knows which 8 they are. They are not the top 8. A checkmark or change of color when read would be the ticket. I'm hoping for eventual healing for this feature of the site.