Canvas will assign your assignment to everyone in your course by default.
You can add more than one section/group to the Assign to field as long as the due and availability dates are the same for those sections/groups.
If you want to set different dates for different sections/groups, click Add below the date pickers, then Assign To the respective sections/groups and add the dates that apply to those sections/groups. This way, you need not create multiple assignments.