Note that the maximum credits include the daily free credit that is available for your version based on how many user licences you bought and any extra credits you bought.
For Professional edition, you can now buy credits until you hit a limit of 1,000,000 credits. Before, the daily limit was 500,000 credits. For Enterprise/Zoho One edition, you can now buy until you hit 2,000,000 credits, up from 1,000,000 credits before.
Let's say you're using the Professional version and have bought 100 user licences. The number of credits you have now is 100,000. The amount is found by adding up 50,000 base credits and the number of user licences times 500. When you hit the maximum of 1,000,000 credits, you can buy 900,000 more credits.