Product Fruits allows you to create tours of any length. However, we've found that limiting the number of cards per tour works best in retaining the attention of your users. Generally speaking, we recommend around 5 cards per tour, but this can vary from application to application.
While there is no strict style guide in how you choose to structure your tours, variety will help keep your users engaged. It may be tempting to create tours using only the "Highlight element" feature, but this tends to make tours feel more one-dimensional. Consider varying the content of your tour using modal dialogues, video/GIF examples, and other preset templates provided by the Product Fruits editor.
Once you've become more comfortable creating tours, there are certain techniques that can be used to bend the 5 card rule. One such method is placing your user in a new environment. At the end of a 4-5 card tour, the Highlight element feature can be used to segue into another section of your application.
Keep in mind, this technique should be used sparingly in order to avoid user fatigue. Whatever approach you choose to take in building, always remember that there are no hard rules in design. Different tour lengths are going to be appropriate for different applications, and the best practices are conceptual- test designs first with employees, get feedback from users, and balance ideas with intuition. You know your application best. Product Fruits is designed to maximize its impact.