You might be surprised to learn that spells to become a witch are not really necessary if you want to practice witchcraft. A witch is not something that you magickally turn into because you want to be one, you become a witch by performing witchcraft.
So instead of looking for spells to become a witch, you just need to find some easy spells for any purpose to try, and cast them. You learn to cook by getting out a recipe book and cooking, and witchcraft is much the same thing. Don't expect immediate and miraculous results. Real life isn't like a Harry Potter movie.
It's a very misunderstood practice, and its often mixed up with the Wiccan Religions. Let me be very clear, you do not have to be Wiccan in order to be a witch. While Wiccans do cast spells and practice witchcraft, you can still be a witch without having the same spiritual beliefs as a Wiccan. Don't let yourself get confused when people talk about Wiccan Spells. They are all witchcraft spells.
So how many spells do you have to perform before you can truly call yourself a witch? Sorry, I have no answer for that. It's sort of like asking how many recipes do I have to try before I am a cook. Are you successful with your spells? That's a big factor in whether or not you can say you are a witch.
And you have to be honest when deciding that. If you just delude yourself into thinking your spell did something that most likely would have happened anyway, then that's not really success.
Ok, I think I've made my point. There is no instant way to become a witch, it's just something you have to learn by doing. Browse through the categories here on the site and find some spells that seem interesting to you. Try them, and you're on your way to learning the art of witchcraft.