Max is for people who build pipelines and dashboards with the agent all day. It includes everything in Pro, plus 5x or 20x more included model usage than Pro, priority access to that usage at busy times, and early access to new templates and platform connectors.
Max 5x is $100 per month and Max 20x is $200 per month, both billed monthly. If you run most of your work on your own model key, Pro is likely enough, because requests made with your own key do not count against included usage.
These are planned prices, and nothing can be bought yet. See what the paid plans add, how usage limits work and the pricing page.