It is worth knowing 12 or 24 are the most common number of words in a mnemonic. However depending on the algorithm, it can be any number.
It can be different numbers than 12 and 24, and it can be different in security with same 12 words or 24 words.
This article from Jameson Lopp gave many examples of bad seed phrases.
How Many Bitcoin Seed Phrases Are Only One Repeated Word?To be clear, the BIP39 specification allows for generation of seed phrases that consist of 3 / 6 / 9 / 12 / 15 / 18 / 21 / 24 words, but for brevity I'll stick to the most commonly used 12 and 24 word lengths.
They are examples of bad seed phrases need to be avoided. It can be only a test as I don't know any wallet softwares create such seed phrases as there is no reports on this issue as far as I know.
Actually the BIP-39 text is very clearly stating that "The
allowed size of ENT is 128-256 bits" which means it does NOT allow generation of any mnemonic with entropy that is smaller than 128 bits (12 words).
What I said above about the "algorithm" was about other mnemonic algorithms out there that allow other word lists with different lengths that can produce a mnemonic with smaller number of words but with the same security. Remember that these words are just encoding of an underlying entropy and as long as that entropy is bigger than 128 bits, the derives keys are considered secure.