Variation with minimum pieces

Hello everyone, among the products on sale on my site, there is a set of knobs, consisting of seven pieces.
I need, however, to add some knobs, depending on the needs of the customer.
How do I set the product so that the base to be purchased must be the complete set of knobs, giving, however, the opportunity for the customer to add a total of knobs according to his needs?
Consider, again, that the set of knobs is not made up of knobs all the same but of six different types, let me explain: in the basic set there will be a smooth knob, a knurled one, one with the words etc. etc.. etc., suppose, however, that the customer has the need to add a knurled knob and two with the words, how do I make these variants product?
Hello and thank you very much