I need to fit a long sentence in a small space and cannot change the font or size.
It will be used for an UI of a ticketing machine in buses to announce how many passengers the ticket is for.
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The most common abbreviation for passengers in the logistics / transport world is probably pax though I don’t know if that’s specifically American English
While pax is certainly used within the industry to refer to passengers, and would probably be understood in the context of a machine display (because the buyer asked for tickets for 2 passengers and the display shows "Pax: 2"), it's jargon.
A fairly standard way of producing abbreviations is to remove the vowels from a word, followed by doubled letters. Using that method, passengers becomes psngrs, which can almost be pronounced. You might consider psgrs, or psgr. Or simply say number! Number does have recognisable abbreviations.
Using something like Ngrams to find relative popularity of abbreviations isn't really possible as they don't appear in books. Notices, which is what is being created here, aren't included. It will be necessary to do research into what is actually used in other machines.
I've looked at previous train tickets I've got and where numbers are shown, they are simply shown as "Adult" and "Child", but that's in the UK.