When an agent asks for this is it an either or, or is the 5k a max word count? If you have a prologue, should this be included ? Some ask for the first fifty pages. Are these word doc pages or book pages, and is there a difference? Fifty pages is up to Chapter six on this book. My first three chapters are a bit shy of 10k, the first two, are 6k. (some do say up to 10k, so alright there)
They are looking for the opening "chunk" of the novel, which is usually defined as 3 chapters, assuming it lands in the 5k-ish range. Should 3 chapters fall short of the word count, no biggie. Should it go over by a lot, they only want the 5k, but cut off at a natural stopping point. Not mid sentence or anything. That would be silly, though nearly everything about the submission process is silly on some level. As for 50 pages, I always assumed that to mean 50 double spaced Word pages or its related software equivalent. Prologue counts as a chapter.