OK, I found the way. Not perfect one, but usable. My form is now limited to the specific maximum quantity of the items (20 in this case).
I needed 20 hidden fields, which gets boolean values defining line visibility option. They are used as conditions for displaying each line container.
What can help much in the future Form package versions:
Possibility to use the expression in condition field - I can define condition based on the one variable instead of 20. BTW - when I tried to edit JSON definition of the condition value, it broke component to the unremovable and uneditable state and I needed to create whole form again.
Possibility to use list / array / dictionary value in the Field key - same as above, now I use one variable for each item on 20 lines, which is very uncomfortable