Your system wouldn't be even in Normal Form 0 if you would store multiple values in one filed.
Redesign your datamodel.
Go from
TABLE account_item
name VARCHAR(50),
account INTEGER,
....
(with no way to store or retrieve multiple values into/from one INTEGER field. You would have to rely on a CHAR or VARCHAR field to store multiple values, you would also have to convert the number to and from a string to store or retrieve the value, how would you access the third value, .... a real nightmare)
to
TABLE account_item
ID INTEGER,
name VARCHAR(50),
....
with primary key (ID)
TABLE account_item_accNr
ID_account_item INTEGER,
nr_account INTEGER,
....
with primary key (ID_account_item, nr_account)
and foreign key (ID_account_item) that references account_item (ID)
For each account number you want to store, you just add one account_item_accNr record that references to the correct account_item record.
Going from your problem to this solution is a basic datamodelling skill. I'd recommend reading a good book about datamodelling or taking a course. It's not that hard to learn, but you really should master it before proceeding.
You should really have a good grab about normalising tables before doing anything in a production environment. Otherwise you will hit the wall and you will hit it hard. A normalised datamodel is easy(er) to expand, modify, update, query, ... , anything else will make it LOTS and lots harder.