Hi, there!
Indeed, on the 2-page template, there’s room for only a single mission. The full SRD offers ways for you to provide more missions after that to the players.
The mission section is what players are supposed to do. There’s a problem that requires immediate action. That’s where you describe it.
I think the best way to understand it is to look at a finished game! Check out this game for example (it’s free and great!)