Try and get a feel for the usual amount of photos that are needed per case study, or ask if they could commit to a maximum and build the design (seems like you have done so) in a way that will respond well and look fine whether there are 2 or 20 images.
Create an a Collection of images. Then include a multi-ref field in the portfolio items Collection. Reference images as need. (Pro tip: name the items in the images Collection with the client’s name at the beginning to speed referencing.)