>>Where is the problem with this option?
If the field of list or multi list type, it is saved as "enum" type of field in the database. Values of "enum" type of field, in turn are saved not as values, but as integers. For example:
USA= first power of 2=2
Canada=second power of 2=4
UK=third power of 2= 8
Greece= fourth power of 2=16
PHP has a PHP_INT_SIZE with maximum value of 2 747 483 648. Which is 31st power of 2. ( 2 ^31).
That is why currently you can't enter more then 31 values.
To fix this issue data should be saved in another type of field ( not "enum" within the database). This iscomplex change, since "enum" type of field has quite a lot of benefits which are already used in other features.
Having other important features to develop ( http://www.oxwall.org/roadmap) we just didn't have time to improve the feature with list/multilist type of fields.