Should each client, brand or location get its own sub-account?

How you use sub-accounts is up to you. The typical use is one sub-account per client. Some people use one per physical location, and some use one per brand so each brand stays separate.

General rule: if there might ever be upselling or cross-selling between things, keep them in one sub-account, because cross-selling across sub-accounts is difficult and it is much easier with all your leads in one account that you can market to and send to different offers. If things are completely separate and you will never cross-sell between them, use different sub-accounts.

If you run several funnels and agents for your own business, you can keep them all in one account. You do not need a sub-account, or an agency, to use the platform for yourself.