When you learn about the cytoskeleton of a cell, you learn that there are three different types of protein filaments in the animal cell (microtubules, intermediate filaments, microfilaments). You also learn the structures that are made of microtubules — centrioles, flagella, and cilia.
As illustrated above, centrioles have a ring of 9 triplets. Flagella and cilia have the “9+2 arrangement” of microtubules.
The magic number you’re just gonna have to memorize is 9.
An easy trick is to remember that centrioles have the 9 triplet arrangement and the rest have the so called "9+2 arrangement" which by the way means “9 pairs + another PAIR (yea that’s 2 for ‘pair’)”.
Not sure how helpful this post would be because it’s such a small detail out of everything you need to know in biology haha… But, I for some reason could never remember which structure had which microtubule arrangement and hopefully this helps somebody!