There are tons of different ways for agencies to ask for submissions. Some places request just a query. Some ask for a query plus a few pages. Some less reputable agencies ask for a query plus a head shot and your measurements.
When setting the submission guidelines, we at Upstart Crow had a lot to decide on. Should we accept queries through email or through an online form? What should we project for response times? Should every writer begin their query with a knock-knock joke? And, most importantly, how many pages should we ask for?
At my old agency, we asked for a query and the first two pages of a manuscript. I found with that setup I could easily spot manuscripts that were completely wrong for me based on the query, and I could usually figure out in two pages if the writing had promise or not, but I found myself requesting a decent amount of material where I was somewhat unsure. I broke it down once and figured out that my request rate for seeing manuscripts was probably about 10% of the total queries I read.
At Upstart Crow, we decided against knock-knock jokes and instead to ask for 20 pages pasted into the body of the email. (To see which 20 pages, refer to this post). I'm finding that 20 pages gives me a great chance to read a good chunk of a manuscript and really get a sense of the writing, the pacing, the plot, and the strength of the characters. Do I know whether I'll sign a book in 20 pages? No, of course not. Do I have a good sense of whether the author is serious about her story? For sure.
For me, the 20 pages makes it more difficult plowing through my submissions pile, since there's more to read, but makes it easier for me to pass on material I may have flip-flopped on before.