Japanese picks a counter to suit the shape of the thing being counted — and the number in front changes sound to fit it. Every counter below shows one to ten, with the readings that change marked in gold.
There is a native series — ひとつ、ふたつ、みっつ — that works for almost anything up to ten, and a Sino-Japanese series where a number goes in front of a counter suited to the thing: 二本 for two pencils, 二枚 for two sheets of paper.
You cannot say いちほん. It becomes いっぽん. Counters starting with h- turn to p- after 1, 6, 8 and 10, and to b- after 3. Counters with k-, s- and t- double the consonant after 1, 8 and 10.
Put 何 where the number goes: 何本, 何枚, 何人. It takes the same sound change as 3 — なんぼん, not なんほん.
The same readings as a table, for looking something up rather than studying it. Gold cells are the readings that change. Click a row to open that counter above.