Wait, they couldn't have done that with Tom....don't you see? The Yeerks would not have let Tom just "go missing" or have a death faked. They would've hunted him down, he was getting higher in the ranks. If he all the sudden went missing or dead, and then another "Andalite Bandit" showed up, they'd put 2 and 2 together. And then they'd kill Jake's family. And maybe Jake in his sleep. Or whatever.
I really think that more members wouldn't have provided much. They were a unit. They knew each other. One more body to protect and try not to leave behind might have been too much. Especially someone who is not like the auxilary morphs. Someone who could make up their mind to go wherever and do whatever they wanted. David was the classic example of what would've happened.
You lost everything, the aliens are trying to control your mind or destroy you, and we are the only hope to save the world. Here is a power greater than anything you've ever known. But you can't use it unless it is to help us kill things. You can't even go anywhere or do anything without telling us. We own you now. And if you don't listen, we will murder your family who has already been taken by aliens...... ok?
I'm glad they didn't recruit anyone else. I wanted them to, but if we wanted the series to be the way it is (classic), the new animorph wouldn't have made a very large impact on the war. And if the new animorph didn't make a significant impact, why add him/her?