How many literary agents should I contact at once?

Well, don't get too ahead of yourself. It is great to think about these issues in case they arise, but chances of you having more than one agent offering to represent your work is small, unless you have something great on your hands. Most writers are lucky just to get one agent interested. BUT, in case you're one of the lucky few, most agents will expect that you're contacting other agents, and if one agents offers representation and you decline because you're going with another agent who's offered, then they see that as part of the job - it happens. If you get offered representation then send an update to the other agents who are considering your manuscript to let them know. They'll prioritize your manuscript and either offer to represent you or send you a rejection. If you get offered representation by one of your top agents then send an e-mail to the other agents asking them to withdraw your manuscript because you've been made an offer. But, I'm also assuming that you're not sending your work to someone you don't want representing you, right? So, the only time you'll decline an offer is if you have more than one agent making offers to you and you need to pick one and decline the others. I'm assuming you already know this, but it's hard to tell from your post.

You need to find that great balance between sending out too few queries and too many. If you send your query one at a time then it might take you a year or more to get through all the agents. Does that seem like a reasonable time to you? Most agents don't respond the next day; they take weeks and sometimes months to respond depending on how busy they are with their clients and how many queries they're getting.

Contrarily, if you send it out to too many agents at once then you can be blowing your chance with a bad query. Depending on how many agents you plan on submitting to, you might want to send it out to about 10 at a time. By doing this, you can revise your query as you continue. For instance, if you get 10 rejections then that can be a sign that your query isn't as effective as you think it is. So, you revise it and send out the newer version to the next 10 agents, and keep repeating the process.

The same is true for your manuscript. If 5 agents ask for a full and all five reject you then it's not your query letter, but there could be something wrong with your manuscript and you should think about revising, before querying the next batch of agents.

Good luck with your novel.

Some agents will request no simultaneous submissions, meaning they want you to submit only to them. More will, if they ask to see your full novel, ask for an exclusive look, meaning you not have other agents reading it at the same time. Most, however, are fine with simultaneous submissions, but you probably don't want to go above five or so at a time. If you end up signing with an agent and then receiving follow-ups from others, just thank them for their time and tell them you're no longer seeking representation. If they've been in the industry for any amount of time, they'll be used to this.

It can be a frustratingly long process finding an agent, but stick with it. Good luck!

It's all right to send queries to multiple agents at once, but if more than one shows interest in you be sure to tell them this. It's not going to make you look bad because after you already have an agent you shouldn't need to find another one.

do a few at a time. see what people say about it and then go from there.