"After you slice bread, there are many crumbs."
Translation:Post kiam oni tranĉas panon, estas multaj paneroj.
Certainly the two correct responses are technically correct. I nevertheless failed to select the one making use of "multe da" because it seems to me there is a strong preference to use "multe da" when describing a singular noun (ekzemple: "multe da akvo") and to use "multaj" when describing a plural noun (ekzemple: "multaj krajonoj"). So my questions are (1) does that preference really exist (even if only informally) in Esperanto; and, if so, (2) how strong a preference is it? In other words, would Duo's correct answer of "multe da paneroj" be jarring to a veteran Esperanto speaker?
Why not "Post vi trancxas panon
Because post is a preposition -- it can be followed by a noun but not by a clause with a verb in it. You need a conjunction for that, and post kiam is what we use in this case.
English "after" can be both a preposition and a conjunction, but not Esperanto post. Compare, perhaps, with English "during" which is a preposition but not a conjunction -- you can say "during the night" but not "during I was sleeping".