is there a hidden option to set different numbers of cells for different folders?

Not exactly

But you can have a different number of cells in each folder.

If you don't want to see the empty cells you can hide them.

So in essence you can have as many or as few cells as you need.

Then only global setting is the size.
That is the same for all the cells on all the pages of Fast Dial once it has been set

Many thanks

but the folder overview display it as it has more cells and not for example 2.

Have you got hide empty cells selected in preferences ?

If you want you can put a logo onto the Folder cell.

There is even a transparent Logo on the site that will make the Folder cell completely invisible.

Please see here

Many thanks