Mitsui Sumitomo and Lloyds Bank (among others, I presume) use 7 digit bank account numbers. It seems that if they have more that 9,999,999 customers, they wouldn't be able to route transactions to the new ones.
While I couldn't find an exact figure for the number of customers this page indicates that Lloyds Banking Group (of which Lloyds Bank is the foremost member) has 30 million customers. Furthermore, Mitsui Sumitomo is, AFAIK, the largest bank in Japan. Japan has a population of 120 million, meaning that if Mitsui Sumitomo has a 10% market share it would have more customers than account numbers.
How do they get around this?