(I had to think exactly how I'd phase all this for the blog post).
I have three that I'd trust with my book (not including one MG reader that is just fantastic. She - and my husband - are termed my 'end readers', which are different.) and see it before anyone else, including agent. Of the three I expect different things, however, and mostly receive them.
One is fantastic for the big picture, the theme, and the motivations. She is harsh, and critical, and I love it. One is excellent for her (isn't it funny, hubs is my only male reader. hm...) on target "this doesn't make sense" bits, and is also unrelenting. The last is perfect for her eagle eye and knowledge of grammar, since mine goes the longer I stay here. My crit group is also great, but they've been excessively busy lately and haven't been able to do much with PoloGRRL.
So going into it, I expect and receive different things from each beta reader. It took me some time to know what to get from each, and I also critique their MS in return. It's a healthy, happy whole which ends up making my books very strong and powerful, as they're all at the same level as I am right now.
And you? How many betas do you have? What do you use them for, or do you get different things from each as well?