Well, a lot of them are not really 'alerts', since the way the board handles quotes and the general laziness of posters here in editing out those whose posts aren't being cited creates a lot of false 'alerts' when your posts are imbedded in other people's quotes, especially when two or three people are playing 'I Touched You Last!!' somewhere.
In any case, your 'Alerts Show All ' pages keeps a list of them all, though it doesn't tell you which ones you've already read. The above thing about imbedded quotes would make it useless anyway. I get sometimes 40-50 between log ins, normal is around 20 on a slow day, but a portion of them aren't really 'new'.