DFC, an aircraft owned by a company need not have a public category C of A simply in order to allow the shareholders to pay for the right to fly it: see Art 130(10) of the ANO.
By the way, it is not strictly correct to say that no more than 20 people can own an aircraft, or shares in a company which owns an aircraft, but, if there are more than 20 shareholders (ie: if each owns less than 5% of the aircraft or the company), then the "private flight" rules in article 130 will not apply if payment is made for the right to fly the machine.