How Many Query Letters can you send out at once/?

You can send as many as you want. To know whether an agent wants an exclusive, read the guidelines on their website. If they say exclusive, I'd make sure to get a time-frame and make sure it's decent; you dont' want only one agent holding onto your query letter or manuscript for 6 months before you hear back. Usually, though, exclusives come if they request the full manuscript, not the query letter.

Other than exclusives, send out as many as you want. I do advise you to send out batches of 4-5. This gives you a chance to revise your query letter if you start getting lots of rejections. If you send out 20 at a time, you might get 20 rejections before you realize a flaw with your query letter and by then you've run through a good portion of agents and many will not reconsider.

The choice is yours, probably the more the better. Once an agent is interested in your work you can discuss contract terms, whether the agent wants an exclusive contract and is prepared to pay for exclusivity etc.

i say you should be able to send out as many as you like. i see it the same as putting in applications for a job. the more you put out there, the better.

you may want to space them a week or so apart, though. that way it gives some time for responses.

added--when you say exclusive, do you mean once you take on an agent? that would depend on the terms of the first contract.