I need to know what is the maximum number of users for a single app. Also, how will I know how many "seats" are left for an app. I currently have 598 users in an app and needed to add additional users.
The number of Users you can add to an Application is defined by the # of Users you pay for in your QuickBase Account. There is usually a minimum for each plan. Once you reach the "limit" you are not prevented from adding additional users; but ahead of your next billing cycle, QuickBase will advise that your Plan needs adjusting before your billing date, otherwise your plan will be adjusted accordingly automatically to accommodate the unpaid-quantity of Users over your paid-number.
If you have access to the "Manage your company name" link on your My Apps page, then you can see how many Users are paid for in your Account versus how many exist. If you do not have this link, you should contact the QuickBase Account owner/administrator to inquire as to this information.