There are two interpretation for this.
Admission open means referring to only one kind of admission open in your institution.
Whereas Admissions open shows enter to number of courses/field
Admission refers to permission given to a person to enter a building, club, school or a place.
Example : No admission after 10 PM.
Admissions refers to the process of taking in students to a college, university or any institute.
If only one seat is available “admission open”.
If two (or more) seats are available,”admissions
open” is correct.
Example : Admissions policy is quite different here in the college.