Hi,
The only limit is the type of the column in the database. Based on your initial version of HikaShop on the website, it could be text or longtext. If it's text, then you'll be limited to a few thousand values. In which case you can switch to longtext for the field_options column (like it is by default since several years now). With longtext, the limit is around tens of millions of values and in that case, you'll have a memory issue and loading delay issue rather than a storage issue. But I doubt anyone will have so many values in one custom field.