Totally normal feels, and congrats! You're going on sub!
Our first rounds are usually about 10 editors. My agent gets a feel, sometimes by talking to the editor in advance in meetings or lunch or other conversations, sometimes she already knows what their interests are or what they're looking for. This is good because otherwise she might be sending to someone who has little or no interest in your specific type of book. You might lose a chance at that imprint all together with that pass, and the agent doesn't want to waste her time or the editor's time. Plus, if an editor feels that the agent is always bringing them stuff they never want/like, they might tend to dismiss future proposals from that agent. The agent should have a keen sense of who to go for, just like querying agents.
You don't want to send the book to a huge amount of editors, just like querying, go in stages. The agent has top tier editors who would most likely go for the book. Sometimes, if you get rejections, there might be a pattern for you to change or revise, and thus implementing that before a second round of submissions. I've always heard to be wary of agents who throws a book out at all their editorial contacts at once.