How many payment submission can I receive with a paid account?

That would depend on the actual payment plan that you subscribe to.

I should mention that our pricing page shows this only for free plan, because the paid plans are not limited to any type of submissions - all of them are treated as same, so you can use the same number of regular submissions, secure submissions and paid submissions per month - for example, as shown on pricing page, at this time, Premium Plan allows you 1000 submissions of any type.