From four realities, first citta needs to be understood. It is the nature that is aware of its object. No other dhamma or nature can know anything including themselves. But citta can know everything possible including cittas.
Citta always leads other nama dhamma and rupa dhamma. A citta arises, it passes away immediately after its arising. Another citta arises, and again it falls away. Next arises and dies out immediately. This kind of uninterruptedness is the manifestation of citta. There are immediate causes for arising of citta. They are cittas themselves, nama dhamma and rupa dhamma.
There are 89 cittas in total. 81 cittas are mundane consciousness and 8 cittas are supramundane consciousness. At another time, citta can be counted as 121 cittas in total. This happens when 8 lokuttara cittas arise when in jhana. These are called lokuttara jhana cittas. As there are 5 jhanas, then there are 40 lokuttara jhana cittas. Together with lokiya cittas 40 and 81 will make 121 cittas in total.
81 lokiya cittas are 54 kamavacara cittas, 15 rupavacara cittas, and 12 arupavacara cittas.
Kama means sensual things that are related to 5 sense doors. Avacara means arising frequently. Kamavacara means frequently arising in kama bhumi or sensual sphere. So do other terms rupavacara is rupa brahma bhumi and arupavacara in arupa brahma bhumi or realm or plane of existence.
Kamavacara cittas are 12 akusala cittas, 30 asobhana cittas or non-beautiful consciousness, and 24 sobhana cittas or beautiful cittas.
12 akusala cittas are 8 lobha mula cittas or greediness-rooted consciousness, 2 dosa mula cittas or hatred/ aversion-rooted consciousness, and 2 moha mula cittas or ignorance/ delusion-rooted consciousness.
