The 'On Change' functionality seems to fit with your requirement, or why would you disagree?
You can have a user enter integer attribute 'Input' with 8, and have the string attribute 'InputDisplayString' changed to '8 text boxes' using an OnChange microflow.
EDIT: Or, if you do not need to have this string stored in your database, you can just use a text element:
EDIT2: Nevermind, I understand your question now. What exactly do you mean with text boxes? And what is the desired functionality? An On change microflow would be the way to go, creating 'textBox' objects depending on the entered number. You can create your custom loop for this, where you create the object, do value-1, and check if it is >0. If yes direct back to create element, otherwise stop.
Much easier would be to just have an 'Add', 'Edit' and 'Delete' button available for the text boxes, this prevents complicated scenarios (for instance when elements are created, filled in, and user then decreases the number etc.)
You could do a same like setup as this forum post: https://forum.mendix.com/link/questions/88601