Puzzle n°49 (*) - Fawn Silver Somalis

- Is this beautiful somali yours ? What is her coat colour called ?   this is the question asked one day by a visitor in a cat show. - This colour is called « fawn silver », the soft orange-beige base colour contrasts with the pure white hair base. It's the lightest colour found in the breed. The conversation went on for a long time. This visitor was very happy to learn a lot of things about somalis... - How many cats do you have ? - Not many. This breeder over there has 20, but I'm far from it ! I prefer to keep a small-size breeding activity, for the cats to be better at ease and to enable more interactivity with them. - But how many ? - Well, if you take any two of my cats, the probability that they both be fawn silver is exactly 50%. - Oh, I see...

Are you, too, able to compute the number of cats at this stage ?

... I'm giving up...