S'il n'y a que 6 colonnes, alors une façon de procéder
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;