You may have forms which were created using applications such as Microsoft Word or Excel. But, you may want forms which are more interactive. PDF forms are the best if you want forms which are interactive. You can easily create interactive fillable PDF forms from word forms or excel forms with the help of Adobe Acrobat DC.
You can create as many fields to fill out as you want in each fillable PDF form. There are no limits as such to the number of fields you can have within a single PDF forms. You just need to make sure that the form is not too long. Otherwise, it would be convenient for users. They would have a tough time while filling the forms. A fillable PDF form with too many fields is not user-friendly.
Adobe Acrobat automatically recognizes the static fields of scanned forms or digital files and converts them into fillable fields which can be populated either manually or automatically. It depends on how the user wants to fill those fields. Filling them manually would be a time consuming process. At the same time, if accuracy is important, then filling the fields in the form manually would be the best option. Otherwise, you can setup a macro to fill the fields in a PDF form automatically.