Policz wystąpienia w wielu kolumnach

Jeśli jest tylko 6 kolumn, możesz to zrobić w jeden sposób

select Number, count(*) From (select Number1 as Number from your_table
union all
select Number2 as Number from your_table
union all
select Number3 as Number from your_table
union all
select Number4  as Number from your_table
union all
select Number5  as Number from your_table
union all
select Number6 as Number from your_table) myTab
group by Number;