What is the filling of the stapler called in English? (you can see it in red circle in the attached picture).
I looked for this word in my native dictionary but I didn't find this word.
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I'd just call them "staples".
A set of staples joined together like this can be referred to as a "staple strip" or a "strip of staples", as in Eran's answer, but English speakers don't use this term very often. Instead, most speakers simply refer to them as "staples" in normal speech.
That doesn't mean you can't talk about strips of staples. You can talk about whether a stapler takes half strips or full strips, for example – the shorter ones take half strips. But in common speech, it's usually most natural to just say "staples".
By the previous answers here, I found the following things:
Checking in google images shows that the boxes that contains them, call them simply staples. (See here for example)
Staple strips, by googling I found mainly as a term that can refer to paper staples, but not as name of product title for them but just generally referring to what it is called. I found also that "staple strips" as a product title name refers mainly to the staples that are not for papers, but for example for thicker things than paper or fences etc. For "paper staples strips" there are only 5 results in google.
Stapler needles is a name that less common in English speaking countries (for example, by googling I found 972 results, in which 46 results in the UK or 774 results in the US for "stapler needles"). I think it can tell everything about this usage.
They are called staples.
I have always said "a row of staples" for the actual staples stuck together that you put into a stapler.
In the US, Swingline is the most known brand of stapler and here is the usage for row of staples
Just wanted to point out that one of the world's largest office supplies companies based its name on these little things:
From a marketing perspective, it has a dual meaning - a "staple" is typically a basic ingredient or material so widely used or consumed that it's considered essential to an everyday process. For example, rice and potatoes are staple foods in many parts of the world, cotton is a staple element of clothing.
By labelling themselves Staples, the office supplies company acquires the linguistic side effect of describing themselves as essential/being sellers of essential things
In India, I have heard them called "stapler pins" most often. See, for example, listings on various shopping sites:
Or various news articles:
‘Indecent obsessions’: A peek into life as a kleptomaniac, compulsive liar - The Hindustan Times, August 20, 2017 (emphasis mine):
Pencils, erasers, sharpeners, notepads, paper clips, even stapler pins – over the next nine years, Dandekar would stockpile stationery he never really wanted, and certainly did not need.
29 stapler pins removed from 10-month-old baby's oral cavity - Daily News & Analysis, May 6, 2015.
As already answered, most people would just say they were "staples":
I need to put some more staples in my stapler.
It may also be called "a block of staples" as they are stuck together.
Another alternative is that they may be referred to as a cartridge. Usually, a "cartridge" is a container with something inside, but the fact that the staples are sort of stuck together with a very fine adhesive makes them more than just "some staples". It is a single, collective object designed to be put into a stapler, and for that reason, I think "cartridge" is acceptible.