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.
It turns out there are 130 valid seed phrases that are the same word repeated 12 times:
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.