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