The term 'clavier' (as used in the original question ) encompasses all keyboard related instruments, from harpsichords through foot pedals to modern pianos and organs.
I'm guessing you mean a piano/keyboard of the electronic type.If so, I suggest 61 notes - 5 octaves - will be enough to keep you going for two or three years or more. You will find that the majority of songs and keyboard music will be covered by this. A lot of 'better' instruments, like work-stations, have an octave transpose button, to take the whole 'board up and down by 2-3 octaves.
A full-size keyboard should have 88 notes, like a proper piano, but as a beginner, you'll not use the top or bottom octave much. A compromise would be a 76 note 'board, to keep you busy for maybe 5 or 6 years, or even for ever !
Check other answers on this site for other aspects like touch, etc. These may become more important to you as you learn to play.A good teacher will guide you to a suitable instrument far better than answers here.