string[] ssn = { "123456789", "987654321" };

var result_set = db.employee.Where(w => ssn.Contains(w.SSN)).ToList();

string ssn = "123456789";

var result_set = db.employee.Where(w => w.SSN == ssn).ToList();

var result_set = db.employee.ToList().Where(w => ssn.Contains(w.SSN));