You can list categories, such as automotive, retail, apartment, etc. This would having to name companies. You could say luxury retail instead of Gucci, for example.
I have no problem listing the names of some of the MSCs I've shopped for on an application. I would not list the names of the end-clients I've shopped. (As far as I can recall, no one ever asked for client names on an application.) Categories are good enough.
Never list the names of clients, only the categories (financial institutions, for example) and the MSCs if you like. I am signed up with so many companies that I usually list the categories I have shopped, and then if they ask which MSCs, I write that I am signed up with 100+ companies. There is no way that they could expect shoppers to write down the name of each and every company that they are signed up with, and it might not even fit in the field provided on the form.