The apps that you build in Fulcrum cannot have more than 1400 elements. The reason for this limit is due to a database limitation. An element is any of the fields, sections, or labels on the left side of the app designer page that you can add to your app.
The number of elements that your app has can be seen in the fields counter on the app designer page.
NOTE: while this limit is rather high, it is not recommended to have an app with 1400 elements. The larger your app and the more complex (conditional logic, data events, calculation fields) is the more memory the app will require. Your app can become large and complex enough that you will run into performance issues when collecting data. You will want to make sure that you fully test your larger apps on the lower end devices that your field team will be using in the field. This includes creating sample records with as many of the fields filled out as possible.
NOTE: Ways that you can work around this limit is by leveraging record link fields or repeatable sections. However, repeatable sections are elements of the overall app. So you can also run into performance issues when you have an app that has a lot of child records in a repeatable section.