The Yeerk pool is a body of liquid where the Yeerks must return to every three days to absorb Kandrona Rays. The Earth Yeerk pool is located in a giant, ever expanding underground complex which spans beneath the entire hometown of the Animorphs. It's frequently attacked, damaged and towards the end of the books, outright destroyed. The Yeerk pool, along with the Kandrona, is similar to a church, or other house of worship to the Yeerks.
In chapter 5 of The Invasion, Visser Three claims "We'll have to build a thousand new Yeerk pools just to raise Yeerks for half this number of bodies." Given the world population estimate at the time, this means one average Yeerk pool could contain either an average or a maximum (depending on interpretation) of 2.8 - 3 million Yeerks. In 1996, this was enough for most everyone in the city of Los Angeles and about a third of what was needed to control Los Angeles county. Of course, it's possible that the pool under their hometown served the surrounding area of a smaller city.