Its not unusual to have a business requirement with 100 fields on a form. K2 doesnt place a limit on the number of fields you can have on a Form. You should be using Form Tabs , Views to lay ou your form design, by doing this you can manage yor screen real estate, and collate and catagorise form data (UI design) . Once you have your UI Layout sorted you can start planning your business / data layer (SmartObjects) and identify the (CRUDLE) methods that are required to implement the data behaviour of your form . Remember performance is important so laying out your UI so that you load data specific to the TAB / view that the user is working with will create a performant solution. So I think in answer to your question your UI design will the number fo fields that you will be displaying on a form. 

Hi,

Though there is no limitation with 100 plus fields on k2 form with tabs, there could performance hit. Also, rules may get so complicated and thereafter to maintain the application. At one point you may feel like to scrap the form. If there are number of Fields I suggest to use .net form. It would time consuming at the beginning but it will be flexible for any maintainability and changes and without any limitations.