1. Invoice Processing comes with default fields extracted as you can see in the documentation link provided above.
It means the model will look for keys that are likely to be "Amount due", "Billing address", "Customer name", ...
But it may happen that the model is not able to find it automatically.
In that situation, using Form Processing will allow you build a custom model where you will draw nouding boxes on the fields you are interrested in during the training phase.
2. There is no limitation in the number of fields extracted, at least not that customers have encountered so far.
We are not charging per number of fields extracted but per page.
So if you have 100 fields in a single page it will only count as one page.
3. AI Builder is a subscription based model so you are charged on a fix amount each month even if you are not using all your credits.