多い
ōi
When customers and mistakes are both 'many'
The picture The same thing is added one at a time until there is no space left to put it. 多い is a count filling the space; 少ない is a count falling short of it.
Where the picture shows up in 多い
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many people
週末はお客さんが多いです。
shūmatsu wa okyakusan ga ōi desu
We get a lot of customers on weekends.
A line out to the door and not one empty chair in the shop
多い normally comes after the noun, as in 人が多い, not before it.
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frequent
ミスが多いので気をつけます。
misu ga ōi node ki o tsuke-masu
I make a lot of mistakes, so I'll be careful.
A calendar with red circles marked one after another
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a large amount
ご飯の量が多すぎます。
gohan no ryō ga ō-sugi-masu
That's too much rice.
Rice mounded above the bowl so the chopsticks won't rest on it
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there was a lot of it
今月は残業が多かったです。
kongetsu wa zangyō ga ōkatta desu
There was a lot of overtime this month.
A month's timesheet with the night-hour boxes filled in solid black
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often the case
電話で済むことが多いです。
denwa de sumu koto ga ōi desu
A phone call usually settles it.
Eight out of ten items tagged with a telephone symbol
〜ことが多い is the everyday way to say 'it's usually like that'.
All of it in one picture
多い is one picture: the count filling the space. People fill it and the shop is packed; mistakes fill it and you get caught every time; rice fills it and you leave some behind. Overtime stacking up month after month makes a heavy month, and cases stacking up turns into 'that's usually how it goes'.