how to get count(*) value in local temp variable in dynamic sql (ORACLE PLSQL)

Maybe different oracle version, but what worked for me was:

...
execute immediate 'select count(*) from ' || p_table_name into l_count;
...

You can achieve it with EXECUTE IMMEDIATE ... RETURNING INTO:

function count_rows(p_table_name varchar2)
  return number
is
  l_count number;
begin
  execute immediate 'select count(*) from ' || p_table_name into l_count;
  return l_count;
end count_rows;