MySQLで列値の出現回数を増やす方法

MySQLの以前のバージョンで変数を使用できます：

select t.*,
       (@rn := if(@ce = customer_email, @rn + 1,
                  if(@ce := customer_email, 1, 1)
                 )
       ) as occurrences
from (select t.*
      from t
      order by customer_email, created_at
     ) t cross join
     (select @ce := '', @rn := 0) params;

MyQL 8以降では、row_number()をお勧めします ：

select t.*,
       row_number() over (partition by customer_email order by created_at) as occurrences
from t;