I can't remember which book off the top of my head, but I know it is mentioned at least once that the Andalites have a bunch of ships orbiting the Yeerk planet, ready to start blasting it at the first sign of trouble. Only a relatively small number of Yeerks escaped originally, but they then picked up approximately a quarter of a million Yeerks to store on board their stolen ship until more hosts could be found. We can therefore assume that most of the original Yeerk population was left behind on the planet.
Alloran did say in book 8 that they were on "the homeworld" but it is unclear what he meant by that. He could have simply been talking about the traitor introduced in book 18. Since it was never mentioned (even at the end of the series) that the Yeerks had made any attempts against the Andalite planet, we can assume that it was an exaggeration or a KASU. The Yeerks always planned to take the Andalite homeworld, but it is implied that would be their next goal after taking Earth, since Earth would have provided more than enough host bodies.
And Ax was stupid in book 18 for assuming that the Yeerks were invading his homeworld just because Visser Three morphed a Kafit bird. It was made clear in the Andalite Chronicles that many young Andalites in the military acquire Kafit birds, so Alloran probably acquired one years ago, as well as many of his other morphs that we see Visser Three using.
