Hi,
Yes, there is a limit. The field where the data is stored has 255 characters and the system stored in it a list of the ids of the products separated by commas. So the amount of products you can select will vary based on the length of the ids of your products.
There are two other nicer work arounds than what you do:
1. You can modify the type of the "field_products" column of the hikashop_field table in your database via phpmyadmin and change it from VARCHAR (255) to TEXT which will allow for around 65000 characters in the column for each custom field instead of 255.
2. Instead of selecting all the products in the field, I would rather recommend to create an unpublished category and select it in your field. Then, in each product where you want to add that field, add that category in the list of categories and it will add the field to the product. In that case, there are no limits on the amount of products you can add to a field.