How many keys are sufficient to learn mostly all basic piano exercises?
Electronic versions came with variety of keys, and versions with fewer keys are cheaper and use less space at home. However it would be stupid to buy something you would later need to replace to progress above a certain level. I am not aware which exercises a teacher is about to give in the future.
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.
The thing about piano is that unlike most other instruments, playing in the extreme high and low registers is not any more difficult than playing in the middle. So there is a lot of easy music that uses the highest and lowest keys.
For classical music, 88 keys is essential. On top of this, any decent electric piano is going to have 88 keys. Shorter keyboards are more the realm of electronic music, synthesis, and occasionally practical considerations (size).
For other styles, you can get away with less.
Even for a complete beginner, having the full 88 notes is desirable because it allows the player access to the whole soundscape of the piano.
Sit a child down at a piano for the very first time, then ask them to imagine a storm; they will be drawn to the lower octaves for "crashes of thunder". Ask them to imagine a tiny bird, or an icy day, and they will be drawn to the "tinkly" notes of the higher octaves.
It was through experimenting with sounds like this on my grandparents' piano that I gained my desire to learn the piano in the first place; in a sense they were my first "piano exercises", and I was able to immediately produce "music" in this way long before I could play music from a score!
