there have another prince between clovis and lelouch ?

I think Lelouch is 8th in line if I'm not mistaken, not sure though! ops:

Humm.. in the 2nd episode he said that he's the 17th successor. If i'm not wrong

Hmm, he says he's the 17th successor, but second in line to the throne. Odd.

By 17th he means that he was to be the 17th Emperor of Brittania. He was the Crown Prince, i.e. 2nd in line to inherit the throne, because his mother was the oldest/first wife of his father, the current Emperor. Apparently he no longer holds this position becasue his mother died, or because he abdicated as a result of his mother's death.

I m not so sure myself actually, but he definatelly wants to kill the others now. He would most likely snatch the thrown too.

do you mean which position is he in the royal blood line?

The current Emperor of Brittania is the 98th one. Lelouch is/was the 17th Prince. So if Lelouch becomes Emperor, he will be the 99th one.