Rabbi
Moshe Chaim Luzzatto ("RaMChaL"):
138 Openings
of Wisdom
Opening 19
The 22 letters
In total, the letters consist of twenty-two different kinds of orders or arrangements (Heb. sedarim), no less and no more, in order to give the lights the power to act.
Having introduced the subject of the letters, we will now discuss their number.
In total, the letters consist of twenty-two different kinds of orders or arrangements... As you have already heard, each different function has its own particular root, and this root is organized into orders or arrangements that are fitted to the root in question and its intended function. The quality and number of levels in each of these various orders or arrangements is an individual matter depending upon the intended function of any given order. In the case of the letters, each individual letter is one of the various different orders or interconnected clusters of lights necessary to bring about action. Each letter is a unique root suited to the execution of the action that has to emerge from it.
...no less and no more... The reason why there are twenty-two is rooted in the first axiom, into which it is forbidden to inquire, as discussed earlier (Opening 15). For these are the levels required for the intended purpose, in order to give the lights the power to act -- so that the lights prepared in the Sefirot can produce their requisite effects, as will be discussed further below.