Extra Scene 4 did contradict some of my earlier theories I admit it. However I don't recall saying there would be 100~300 girls who would WANT to become claymores. Rather the numbers have very little motivation to carry on living in most cases having lost their family and sometimes their whole village for reasons we don't need to know whether it be war or yoma attack. I think you misunderstood my point, the idea was they don't get a choice in the matter. Many of them probably were sold to the organization as slaves meaning they either become claymores or allow the organization to do away with them however they pleased.
Heres the basic idea of what I'm thinking after reading ES4. Yoma flesh is implemented into their system in the early stage of their claymore trainings. Their numbers can still be 300~100 question then is whats the survival rate of having a yoma flesh inside you? How many live and become Claymores while others either die or become claymore failures like Clarice. I'd assume they still cut down the numbers severely from 300~100 to 10 or if not 20 at most. The propaganda seems to be Survival of the Fittest. They only want the strongest warriors. Also fun fact, the group shown when Claire graduated doesn't seem to show Elena, Claire's friend whom she had to kill in I think chapter 2 or 3. Unless that other short haired claymore was Elena it leads me to believe what we've seen on ES4 is just one group out of many.