It's used multiple times in the UI, in different lists, like: users in the dialogue, users in your project.
Example:
Tom, Bob, Linda, and 1 other.
Tom, Bob, Linda, and one other.
For all the other numbers we use numerals:
Tom, Bob, Linda, and 5 others.
Tom, Bob, Linda, and 20 others.
If we use numerals for numbers 2 and higher, should we spell one, or use the numeral 1?
AP style says "spell out whole numbers below 10, and use figures for 10 and above".
Chicago Manual style says spells out numbers below 100, or as an alternative rule to use AP style instead.
Nielsen Norman Group says, basically, to heck with the old rules; everybody just skims online anyway so use figures for all specific numbers, because people notice those more. (hat tip to @Wanda for this one, which I had not seen before)
There's a reason these are called "style guides" and not "rulebooks". Be consistent, and you'll be fine.
(Personally I'd use numeric figures throughout for this case, because it's more easily distinguishable from the names in the list, and would make internationalization somewhat easier... but if pressed to change my mind on that I'd fold pretty easily.)
If there's enough room to write either of "and 1 other" or "and one other", then (except for very long names) there's enough room to just include that other name itself...
✗ Bob, Ted, Alice, and 1 other
✓ Bob, Ted, Alice, and Linda
...so most of the time you won't need to start hiding values until there are at least two too many to show.
✓ Bob, Ted, Alice, and 2 others
The standard for 'traditional' copywriting is to fully write any number under 10, so one, two, three and so on. However, when it comes to digital interfaces, it's much easier to recognise and process a numeral, than it is to read the written equivalent. Yes, it's stylistically incorrect to say '1 other', but in terms of usability, it has preference.
There's a few reasons:
Numerals often stop the wandering eye and attract fixations, even when they're embedded within a mass of words that users otherwise ignore. [...] Sometimes people are looking for specific facts, such as a product's weight or size, so product pages are certainly one place where you should write numbers as numerals. But even when a number doesn't represent a product attribute, it's a more compact (and thus attractive) representation of hard information than flowery verbiage.
And
The shape of a group of digits is sufficiently different from that of a group of letters to stand out to users' peripheral vision before their foveal vision fixates on them. 2415 looks different than four even though both consist of 4 characters. (As the previous sentence shows, stating the number of characters as a numeral makes it stand out.
Writing for digital mediums is simply different from print. This Nielsen article contains a lot more interesting tidbits to strenghten this approach to online writing.
In your case you're dealing with lists and notifications. This requires scan-ability for your users, making the use of numerals far superior than words.
Grammarly has a nice article about this. It might depend on the language your website or software is in, however.
To quote them:
It is generally best to write out numbers from zero to one hundred in nontechnical writing. In scientific and technical writing, the prevailing style is to write out numbers under ten. While there are exceptions to these rules, your predominant concern should be expressing numbers consistently.
As they mention, consistency is key.
They also mention this:
When a number begins a sentence, that number should always be spelled out. That said, writers often choose to restructure their sentences when the numbers become cumbersome for the reader.
Which might create a conflict in your case. When you want to display a number of users somewhere, I would personally go for '5 users' and not 'five users' as the number is the most important part of this 'sentence' and it won't become cumbersome for the reader. The same goes for '1 other' as it is more consistent to use numbers for all 'others' and won't be annoying to read for the user.
In Germany, there was an old typographer's rule saying that numbers from 1 to 12 should be written as text and numbers from 13 onward are formatted as digits.
However, this rule is outdated. In prose text Duden encourages to spell even more numbers, especially short ones like "fifty" and "hundred". In scientific text, Duden recommends using the digits if you want to draw attention to that number. E.g. if it's important that a bicycle has 3 wheels instead of 2, you would go for the digit.
Source: German Duden
I agree that numbers 0-10 should be spelled out, and when beginning a sentence. I do think @Wanda has an interesting point that's definitely worth investigation.
When I have questions of this nature, I tend to rely on Strunk & White's "Elements of Style" or "The Chicago Manual of Style" whenever possible...
One aspect that hasn't been mentioned in the other answers is if it can cause confusion. Since the 1 looks like an l in some fonts, can the phrase be ambiguous? Can people read this as "l other"? Well in this case, obviously not – l wouldn't make sense, but it's something to keep in mind.
I remember a situation where it was necessary to change the text "0K" (as in, disk space) to "zero K", because people kept thinking it meant "OK".
It's important to not fixate on a hard rule for numbering, and the decision should be based on style, readability and purpose. For example, assuming we employed the rule that numbers below 10 are to be numeral, while those above 10 are written, then we'd end up with awful sentences like: "There were three children walking and 11 children running". Please ensure that whatever you choose, there is consistency with how the unit is quantified, and how reporting is done in the rest of the context. Consistency is the single most important consideration.
