TOP X function

Hi
I am a new user, and I don't know where to set top 50 if I don't want just top 10 ?

you mean the top 10 member posters ? just go into your admin panel and the options link will have a setting for that...

Can you tell me exactly the selection ?

in admin panel -> change options -> memberlist options where you will see

memberlisttopposters x

On the 'Display top x posters' , this option allows you to specify a value for x.