When you're talking to an agent: ONE is the correct number.
Here's why: We get a lot of email. I don't take e-queries and I get 175 emails a day right now (not counting the email from this blog which is in a different mail server program). I use a mail program that allows me to color code things: red for current clients; blue for editors; pink for poodles; green for (CLUE) prospective clients. If I'm reading a partial or a full from you, you're not choppped liver, you're GREEN. The drawback is, I use the email address you put on your query letter and if you reply to me with a different address, you're back in the basic black slush. Even if you leave my subject line intact I might miss it, and I'll for sure be slower in responding.
This is not good. No no.
Pick one email address. You might consider having a separate mail program just for your book biz, and having a default address to send things.
This particularly applies if you have mail on your website.
This isn't some sort of "automatic no" if you don't do it. It just makes life easier. And it means I can get back to you with the kind of speed we BOTH want.