There are two ways to approach it 1) perform multiple find request search or 2) using a relationship.
I suggest using a relationship. It seems easier to users to type a number then hit return and type another number, rather than typing a number, comma, space and another number. So just create a global text field so Users can type into it as multiline then create another table occurrence of your table. Relate them as:
Your table::global = selfjoin::serial
You can then display a portal of the related records' container (basing the portal on selfjoin) or you can Go To Related Records [ match only related records ; match current record only) and get related record from selfjoin. If you must stay with 123, 456 as one line (and want to use a relationship approach) then you will need to create a calculation to convert them to multiline as in first example.
If you wish to perform a search instead then you will need MiddleWords() to create multiple find requests (or GetValue() if using multiline . A multiline relationship would be simplest and probably easier for your Users.