Overall it is pretty nice. The only thing I would do is to constrain aantal planken so that users cannot set this to 1 as it will break the family. This can be done with a control parameter that reads something like 'if(aantal planken < 2, 2, aantal planken)'. This could be expanded to force a maximum number as well by simply adding an or to the statement but that may not work out well for this family. So the control parameter would control your aantal planken array and the original aantal planken array would still be where people type in their desired number of shelves, however, the control would work to prevent users from specifying something that will not work.

And besides the control parameter "if(aantal planken < 2, 2, aantal planken)" you could create a yes/no parameter using the formula "if(aantal planken < 2) and have that display a warning to the user, i.e. model text saying:
aantal planken must be at least 2
(Then at least the user will know that they did something wrong) :beer:

Thanks for the feedback, I will put it in there. It's somewhat dated (guessing 2-3 years old) and didn't have that knowledge back then. That's the problem with designing families, they get outdated as your knowledge increases... but hey, works for me (and the guy I designed it for) because we both know the limitations.

Don't have the time right now but I will pos the updated version soon... Maybe there's even a blog post in it on how these constraints work.