Hello,
You can create your own view and chose to display how many annoucements as you like, or you can modify the existing view.
To create a new view click the Announcement heading and then click modify settings and columns, scroll down to Views. There you can either create your own view or modify the existing.
Once you have modified or created a new view, to apply it on the webpart on the frontpage you will have to modify shared webpart (Announcements) and then under Selected view chose the view you want to have.
Each time you make a change to a view, you will have to re-apply it on the webpart on the first page.
Best Regards,
Thomas