Hi,
There is no limit on the amount of custom fields that can be displayed.
However, HikaShop won't display the custom field if it's restricted to other categories than the categories of the product or to other products.
Also, if you don't set anything in the custom field for the current product in the backend product edition screen, then the custom field won't appear there.
And finally, if the product page display setting of the custom field is turned off, it won't display there either.
These are the only reasons I can think of.