The Sending spell says:
You send a short message of twenty-five words or less to a creature with which you are familiar.
(Note: this question ignores the actual practicality of using Sending in the following ways, or whether the DM would overrule it. It's just about what is theoretically possible RAW)
There is no definition of "word" in the spell description. If a word is simply a chain of letters without spaces:
Do onomatopoeia such as "pstpstpstpstpstpstpst" and "aaaaaaaaaaaaaaaaaaaaa" count as words ? Do made up words such as "shkarliek" or "wekhbda" count ? If made up words are allowed, then what prevents the caster from saying that "iwanttoorderacustardcupcake" is a single, made up word ? (obviously, this is abuse and would be shut down by the DM, but as I have said already, this questions is about what is theoretically possible)
Do abbreviations made of initials count as a single word or several ? For example, is MIA (missing in action) a single word or 3 ?
The creature hears the message in its mind
indicates that the message has to be "hearable" so something completely unpronounceable like "dfjvwdnkjwnljcnqekjfbkjeqnk" is not valid.
The spell enables creatures with Intelligence scores of at least 1 to understand the meaning of your message.
I believe the intention of this is so that the caster can say the message in any language, and the recipient will understand regardless. But there is nothing (as far as I can tell) preventing the caster from assigning a meaning to a made up word.
Since it is not worded as "the meaning of your message, if it has one", I suppose the message must be meaningful ? Meaning, as defined by Merriam-Webster is
the thing one intends to convey especially by language
(emphasis mine)
Especially, not only. So meaning can be the thing one intends to convey by made up words.
Even if we rule it as only by language, as far as I can tell, there is nothing preventing the caster from making a constructed language that is extremely word efficient and using Sending to send something in that language that is under 25 words, but is a lot more than that in Common.
The only other limit I can find in the spell description is the duration of the spell (1 round which is 6 seconds) meaning that the message must be said in a maximum of 6 seconds.