You can query as many as you'd like at one time. That is, the query letter and/or sample chapters. However, as soon as one says they'll take a look, you should consider the work off the table to anyone else, and may have to send letters back to other agents who request a look-see, telling them it's under consideration. So, if you don't feel comfortable with that, you may want to stick to one at a time. I tended to be a little more aggressive when I was doing my agent search, because I knew that for about every ten queries I sent out, I was only likely to get 1 request, if even that, due to my newbie status.
Whatever you do, don't simultaneously submit. That is, don't allow your full manuscript to sit on more than one desk at a time. It's unprofessional, discourteous, and agents *do* talk to one another, especially in specialized fields like humorous fantasy or historical romance. You don't want agents X and Y to end up at a conference somewhere together talking about this manuscript they're reading and realize it's the same piece of work. Agents aren't terribly likely to get in a bidding war over a new author, so making two people do all the work of reading and considering, only to tell one "no thanks" after all that, is a great way to burn bridges. Especially if things don't work out with the agent you choose first. The second agent might not be as likely to consider you a second time because a) you've shown yourself to be a little capricious, and b) you've shown yourself capable of going behind someone's back when it suits your needs.
So query as many as you like, but err on the side of caution when it comes to submitting the entire ms. And if an agent requests the entire manuscript up front, consider it submitted for consideration, and wait on all the others. You may want to leave these agents to the last anyway. Query letters and sample chapters are cheaper postage than the whole thing.
