Not quite...
Switzerland's animal welfare laws recognize that some cats prefer to live on their own, some need to live with their conspecifics. Cats do not automatically fall under the 'social' species.
Most rescues will have already determined if a particular cat needs company or prefers to live on his own - speak to the rescue, tell them all about your living situation, what you are looking for in a feline friend, and most importantly what you can provide - and be open to their guidance.
You can read up on the animal welfare wrt cats here:
http://www.bvet.admin.ch/tsp/02242/index.html?lang=de
Under 'Katze und sich' you will find the guidelines for keeping cats (accommodations, social contact, feeding, inside-outside, etc.), and under Katze und Mensch' talks about your responsibilities as a cat owner.
You can also download the broschure 'Katze Richtig Halten' in pdf format by clicking the link.
What you have heard about adopting at least two animals does indeed apply to other species, but not to cats or dogs. For cats and dogs (and other 'non-social' ) species, social contact appropriate to the individual is anchored in the TschV.