The reason we count ten sefirot is that the sefira of Tiferet encompasses six sefirot, which are Ḥesed, Gevura, Tiferet, Netzaḥ, Hod, and Yesod. Accordingly, they amount to ten. The underlying reasoning for this structure is discussed in the Introduction to the Zohar, in the Marot HaSulam commentary (p. 5). These five levels, Keter, Ḥokhma, Bina, Tiferet, and Malkhut, can be discerned in every emanated being and each creation.
This parallelism applies to the totality of all the worlds, which are five worlds known as Adam Kadmon (Primordial Man), Atzilut (Emanation), Beria (Creation), Yetzira (Formation), and Asiya (Actualization), and which correspond to the five levels of Keter, Ḥokhma, Bina, Tiferet, and Malkhut. The parallelism applies equally to the smallest entity in existence, in which we can discern that its head is Keter, from its head to the chest is Ḥokhma, from the chest to the navel is Bina, and from the navel and below is Tiferet and Malkhut.3Here, the author of the Sulam references the notion that every entity in existence has an underlying structure formed of the sefirot.
The sefirot are the foundation for a more complex structure called a partzuf, which is described as having a head and body parts that correspond to different sefirot. The topic of the five worlds is explained in Petiḥa LeḤokhmat HaKabbala (see sections 6–10).