Many authors I’ve spoken to have different ways of sending out queries. Some do it once a day for a week. Once they start getting rejections they send out a new query per rejection. That way they always have the same amount out there. Others have their list and just query everyone they possibly can. Others send out in groups. Five to a dozen seems to be the common set. Once nearly all of them return they send out the next set.
But, no matter how you query, just send those letters out. At least, that’s what I keep telling myself. 🙂