That's kinda hard to say without knowing a little more about the searches you're performing, but I will say:

1. If you're performing frequent searches, expect to have frequent document reads. That's just going to be a fact of life
2. Consider adding limits to your queries and paginating your results -- make sure your client isn't retrieving more records than your users actually need. This can be especially important when the size of your collections grow.
3. I'm assuming you're asking because of cost, and my general advice here is to beware prematurely optimizing for price. I've seen a lot of really gnarly code on StackOverflow where developers have spent hours creating unstable solutions that are really hard to debug in order to save, like, 10 cents a day. Obviously, you want to avoid doing really bad things like reading in an entire collection when you don't need to, but remember that your time costs money too!