OK, because I just can't leave things alone..apparently it's a setting in usergroups..so we would have to get @Deoan Kakarot or @Barmacral Tigana to take a look. It could be that we have it set to no maximum.
But keep in mind..the only people who will get alerts are the people that have alerts...
I think it's higher than 5. I looked around and couldn't find it - once I have some time I'll dust off my google skills :)
(The 5 is the maximum number of people in a conversation)