The word kalāpa simply means group - hence group of material phenomena in the case of material phenomena, and in the Visuddhimagga XX, 32-43 there are many details about the Octads (i.e. the mininum number of the rupa in one group of kalapas and the various causes for rupa) and enneads.
and so forth - just as explained in the Abhidhammattha-sangaha
XX32 Cittasamuṭṭhānaṃ nāma tayo arūpino khandhā, "saddanavakaṃ, kāyaviññatti, vacīviññatti,
32.(2) What is originated by consciousness is the three other immaterial aggregates and the seventeenfold materiality, namely, the sound ennead
As I think you know, Buddhaghosa edited the already extant ancient Commentaries. And the Vism was far earlier than the Abhidhammattha-sangaha - yet the ancient Bhikkhus long after Buddhaghosa (those at the time of Anuruddha and later ) greatly respected the Abhidhammattha-sangaha as it faithfully follows the word of the ancients.
Why do we have people doubting kalapas or other aspects of Dhamma these days. This is all predicted. Even we are only 2600 years from the time of the Budddha - just a 100 generations - it is not long, yet the Dhamma is declining so rapidly. The Abhidhamma is still here to be understood for those with the courage to see its truths- and it has the reinforcing of the Commentaries, so helpful in explaining knotty points.
I can recommend NIna Van Gorkom’s book on materiality attached here.
the_buddhas_teaching_on_physical_phenomenon.pdf (261.7 KB)