個
ko
If it fits in your palm, it's 個
The picture A compact little lump that sits in your palm — not flat, not long, just a solid thing. If it clumps together in your hand, it's 個; if it's flat enough to stack, it's 枚.
Where the picture shows up in 個
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a lump in the hand
たまごを六個ください。
tamago o rokko kudasai
Six eggs, please.
Picking eggs up one at a time and setting them into the carton.
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the price of one lump
この石けんは一個いくらですか。
kono sekken wa ikko ikura desu ka
How much is one bar of this soap?
A bar of soap resting on your palm while you check the price tag.
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a lump you swap in
トイレの電球が一個切れています。
toire no denkyū ga ikko kirete imasu
The bathroom light bulb has burned out.
Twisting the blackened bulb out and screwing a fresh one in.
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the lump left over
おにぎりはあと一個だけ残っています。
onigiri wa ato ikko dake nokotte imasu
There's only one rice ball left.
One rice ball sitting alone at the bottom of the box.
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a limit on lumps
お預けの荷物は一人二個までです。
oazuke no nimotsu wa hitori niko made desu
Two checked bags per person.
Two bags lifted onto the check-in scale, one after the other.
Flat things (paper, tickets) take 枚, long thin things take 本 — 個 is for the compact ones.
All of it in one picture
Eggs get picked up one at a time, soap is priced by the bar, a bulb is swapped whole, one rice ball is what's left, and luggage is counted in pieces. Every one of those scenes is something you could weigh in your palm. 個 never asks what the thing is for — only whether it holds together as a lump you can grip. So even when the proper counter escapes you, anything graspable can be 一個.