In general in standard SAP the maximum length of the field "Bank account number" is 18. In case you need to enter more than 18chars of "Bank account number" as is the case with China etc then you can follow the procedure as mentioned in note 1585003.
1585003 - How to enter a long account number
A long account number can be subdivided into the basic part (18 characters) and the remaining characters part, hard-coded in the conversion modules (example: Russia, China, Poland).
Example:
CN - China
First 18 positions into the field Account Number, the remaining positions into the field Reference. No general rules for bank number usage. The SWIFT code can be used as the SAP Bank Key.
Examples:
Bank account number: 31014005000220005842
SAP:
Account Number: 310140050002200058
Reference: 42
Bank account number: NRA 622 202 040 800 391 9953
SAP:
Account Number: 622202040800391995
Reference: 3
(the code NRA stands for 'non-resident alien' and should not be necessary for the payment)
For you information purposes, here is a more detailed explanation:
This field is represented by data element BANKN which is defined as CHAR18 and this is part of many databases in the whole Financials and it's also database key. Extension of any database key field is very complicated because all stored data must be converted. This has to be done for all tables using data element BANKN as database key field as well.
You may also refer note:
96624 - DME for bank account numbers with more than 18 digits
115002 - DME for house bank account numbers w/more than 18 digits
Regards,
Sonal Chaudhary