You've certainly received excellent advice from everyone. I will only add a couple of things. When some writers submit to editors or agents who accept simultaneous submissions they omit saying they are enclosing a simultaneous submission. Personally, I think it should be said, but that's your call.
As has been stated, submit to as many or few at one time as you feel is proper. I'd suggest you stagger your submissions and send a few (4 to 8) every 2 to 3 or 6 to 8 weeks. I particularly say this because the entire process is new to you, and any personal feedback you may get from editors and/or agents may help you strengthen your pitch.
It's most important that before sending out anything you research potential sources. Do they accept first time authors? Do they only work through agents? Do they publish the type of book you have in mind -- for instance, if you wrote a nonfiction book and they only publish fiction, ignore them. Lots more to learn before you start to submit. Suggest you go to the library, and online, and learn about the entire process.