Hi,
There is no limitation in the number of items you can put in the values.
However, these values are stored in the database in the column field_value of the hikashop_field table which is limited to a certain number of characters by MySQL.
For new installs of HikaShop the type of that column is "longtext" and thus you're likely to never reach the limit of the characters. But for old versions of HikaShop (which you maybe updated), that type is still the old type, which is much smaller.
So you'll want to edit that column in your phpmyadmin and change its type to longtext and you'll be able to add more values.