Dear Bruno,
Thank you for the message.
The 3D shading tool is designed to calculate shade losses for a residential rooftop system, where trees, neighboring buildings or other nearby objects shade the array. These systems typically have just a few strings, so the limit of 8 works for those.
If you using the 3D shade calculator to model self shading caused by neighboring rows in the array rather than the effect of shadows from nearby objects, you can choose a "Self shading" option on the Shading and Snow input page instead of using the 3D shade calculator.
Otherwise, the best you can do is to divide the array into subarrays, in your case, perhaps three subarrays of 8 strings each.
Best regards,
Paul.