Postgres: return count with name of table and field by reference

I need pl/sql return count of the table .. pass by reference the name of table and name of field

i´m using:

-- Function: fn_post_gera_chave_numerica(character varying, character varying)

-- DROP FUNCTION fn_post_gera_chave_numerica(character varying, character varying);

CREATE OR REPLACE FUNCTION fn_post_gera_chave_numerica(
    var_nm_tabela character varying,
    var_nm_campo character varying)
  RETURNS integer AS
$BODY$

--1o. Parameter: Table
--2o. Parameter: Fields

DECLARE
        var_sql    character varying;
        var_id     numeric; 
BEGIN

    var_sql:= 'SELECT into var_id max('|| $2 || ') FROM ' || $1; 

    execute var_sql;
        
    RETURN var_id;      
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION fn_post_gera_chave_numerica(character varying, character varying)
  OWNER TO postgres;


SELECT fn_post_gera_chave_numerica('tblcadlinhacelular', 'ccodigo');