You can do a couple of things. You can send a query to every agent ever and hope that one of them likes your manuscript…or you can save yourself time and postage.
Pick a top ten to query. Email / snail mail these ten agents. Wait a day (or week) depending on your schedule and your ambition level, then query ten more.
Just like you don’t want to send out a thousand queries at once that are impossible to keep track of, you don’t want to send out one at a time. If you send out one, and wait for a reply before you send out another, you’ll be a hundred years old and still not published.
I stayed organized by setting up special email folders to keep track of my queries, rejections, and partial/full manuscript requests. I was able to access information about a certain agent in seconds because I did a little pre-query planning.
If you’re super organized you can make a spreadsheet with the agents name and query status (sent, rejected, requests). That may be helpful for later on if this manuscript isn’t quite up to par, but your next one is. You can look at your spreadsheet and remember that so and so requested a partial, but ultimately passed. Include that info in your new query letter. Remember, getting published is a marathon, not a sprint. Save yourself wasted time and effort by planning ahead.