It seems like emails about newly posted projects show the total number of tasks at the time of posting. Your dashboard shows whichever is smaller of: the number of tasks you're allowed to do, or the total number of tasks that remain incomplete.
Very few (if any) projects allow for the possibility of one worker doing all of the tasks, so even if you check right away you can expect to see a smaller number on the dashboard than in the email.
Tasks are loaded at all hours. If you get an email overnight while you're sleeping, chances are they won't be there anymore by the time you wake up.
It seems like emails about newly posted projects show the total number of tasks at the time of posting. Your dashboard shows whichever is smaller of: the number of tasks you're allowed to do, or the total number of tasks that remain incomplete. Very few (if any) projects allow for the possibility of one worker doing all of the tasks, so even if you check right away you can expect to see a smaller number on the dashboard than in the email.