I know that Federer won a Junior Grand slam in past and then later he become number one. But how many of Junior Grand Slam Winners make it to pro tennis and reach top 10?
It is too broad to answer without specific timeline. But with Tennis Pro Guru article about chances to make TOP 10 singles ranking for Junior Grand Slam Winners, 16 of 70 boys and 15 of 68 girls made it to TOP 10 during the time period of 1997 – 2016.
So, with the help of the linked article we can conclude more than 20% Junior Grand Slam Winners are likely to reach TOP 10.