I think any of us who have and are sending things out remember that first batch of query letters we sent out, no matter the genre, non-fiction or fiction. We were so sure that we had "it" that we sent out one query. We got that SASE back and ran all the way to the ouse to open it or maybe we torn it open right there at the mailbox.
"I've done everything right, this is it, I wonder how big my advance will be, maybe they will make a movie out of it. I wonder what I'll wear at my first author signing?"
Then we read the line, "I'm sorry but your XYZ doesn't' fit our needs at this time." Instant deflation. And the fall was hard very hard, maybe even broke a few bones on impact.
You can be doing everything right. Have the perfect query, the perfect MS--but it didn't strike a cord with the agent or editor, or that agent didn't feel that they could sell it, or it didn't really meet the needs of the agency you sent it to.
I sent out 45 queries before I landed my agent.
And a memoir? Very hard sell, unless you are someone famous, or infamous in some way, very hard to sell indeed.
I did this with my queries, I got 5 ready to send out, put them in envelopes and sealed them, then as soon as I got my SASE back before I even opened it I dropped those 5 in the mailbox. Then I opened the the SASE. This can work with e-mail as well, save them in drafts, and before you open the e-mail fomr the agent, send them out.
What will happen if you open the letter and find an offer to rep you and then someone else says they want it too? Then you have a choice to make.
As to when do you know the stack of rejections is large enough? When you run out of agents that rep your subject matter, then you have collected enough.
Shawn