Oracle 21c

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create table gp(a int, b int, c int);
begin
insert into gp(a) values(1);
insert into gp(b) values(2);
insert into gp(b) values(2);
insert into gp(c) values(3);
insert into gp(c) values(3);
insert into gp(c) values(3);
end;
/
1 rows affected

select *
from xmltable(
        '/ROWSET/ROW/*'
        passing
        dbms_xmlgen.getxmltype(
            (
            select 
                'select '
              ||listagg('count(*)-count("'||column_name||'") as "'||column_name||'"',',')
              ||' from '||upper('gp')
            from user_tab_columns 
            where table_name = upper('gp')
            )
        )
        columns
            column_name varchar2(30) path './name()',
            cnt_nulls   int path '.'
        );


COLUMN_NAME
CNT_NULLS


A
5

B
4

C
3