var supportcaseSearchObj = search.create({
   type: "supportcase",
   filters:
   [
      ["employee.internalid","anyof",[219,8,10]]
   ],
   columns:
   [
      search.createColumn({
         name: "internalid",
         join: "employee",
         summary: "GROUP",
         label: "Internal ID"
      }),
      search.createColumn({
         name: "internalid",
         summary: "COUNT",
         label: "Internal ID"
      })
   ]
});

I want support case count assigned on employees and it's working fine, But I also want the employees with count '0' if it is not assigned to any but I passed in the filter. How can I achieve this?

Not sure if this is what you are asking for but the following would return those that match those ids and those unassigned.

[
  ["employee.internalid","anyof",[219,8,10,"@NONE@"]]
]

This search is for support cases. You have to start the search from the employee record type if you want to include employees with no cases.

thanks we don't have any link from emp record to support case

You won't be able to search (or query) for the absence of Support Cases by searching on Support Cases alone; you'll have to combine a search of the Employees with your search of the Support Cases. In SuiteQL, you might be able to do that with a subquery or similar struct.