At a minimum, a portfolio shoot would be four rolls that would yield eight shots. Legitimate modeling agencies maintain referral list of experience photographers who can successfully provide images useful to models. Of course the ironies of the business states that a model with great potential will have an easier time finding better photographers to work with for less expense. I devote a small amount of my shooting time to developing my portfolio as well, but when I do shoots I have a broad choice of models often supplied by agencies and can afford to be selective. A model might be able to eventually fill out a portfolio with well achieved images from professional photographers by free testing, but they are at the mercy of busy schedules and the abundance of other hopeful models.
Models can seize the control of their portfolios by paying for some shoots.