In the abstract module you have the possibility to use split fields instead of one text field for the abstract text.
Via the page editor -> abstract data -> right mouse button on the abstract text field you can choose to use split fields in the 'Field properties' tab.
After checking the box at the split fields, the following 'Split fields' tab will be added:
Via the 'Add field' button you can add more fields. Each field can be required and can have a minimum or maximum length.
Setting a minimum number of characters for a split field will not make this field automatically required, so you can determine that a field must not be filled in, but if it is filled in, only then a minimum number of characters is required.
You can choose weather min/max will be in characters or words. Using the trash can will delete a field.
This is how the author sees the fields:
Please note that you can only change the setting to use or not use split fields when no abstracts are registered.