In Japanese, it uses different pronunciation to represent “piece” for different type of things.

For Human – How many person (Nan nin)
Hitori (1 person), Futari (2 persons), Sannin, Yonnin, Gonin, Rokunin and so on

For Animal – How many (Nan Biki)
Ippiki (1 animal), Nihiki (2 animals), Sanbiki, Yonhiki, Gohiki, Roppiki, Nanahiki, Happiki, Kyuuhiki

For papers, clothes, etc in piece form – How many pieces (Nan Mai)
Ichimai (1 piece), Nimai, Sanmai, Yonmai and so on

For books – How many books (Nan Satsu)
Issatsu (1 book), Nisatsu, Sansatsu, ……. Hassatsu, Kyuusatsu

For sticks (ie. cigarettes, tree, pencil, etc) – How many sticks (Nan Bon)
Ippon (1 stick), Nihon, Sanbon, Yonhon, Gohon, Roppon, Nanahon, Happon, Kyuuhon, Juppon

For cups, spoons & glass – Hom many cups / glasses / spoons (Nan Bai)
Ippai, Nihai, Sanbai, Yonhai, Gohai, Roppai, Nanahai, Happai, Kyuuhai, Juppai

For floor / level – How many floors / Which floor / level (Nan Gai)
Ikkai, Nikai, Sangai, Yonkai, Gokai, Rokkai, Nanakai, Hakkai / Hachikai, Kyuukai, Jukkai

For time – What time (Nan Ji)
Ichiji, Niji, Sanji, Yoji, Goji, Rokuji, Nanaji, Hachiji, Kuji, Juuji

For minutes – How many minutes (Nan Pun)
Ippun, Nihun, Sanpun, Yonpun, Gohun, Roppun, Nanahun, Kyuuhun, Juppun

For rounded things, pieces form – How many pieces (Ikutsu or Nan Ko)
Hitotsu, Futatsu, Mittsu, Yottsu, Itsutsu, Muttsu, Nanatsu, Yattsu, Kokonotsu, Too
OR
Ikko, Niko, Sanko, Yonko, Goko, Rokko, Nanako, Hakko, Kyuuko, Jukko

Hope it is not too confusing 😀