"How many cookies would Marcus like?"
Translation:Quot crustula Marcus velit?
"Quanti" (genetive of "quantum") is used for asking about a price.
"Quot" is used when asking about a quantity.
In a broader context than this course, "quot" is for quantity and "quantum" for moral/monetary... value: "Quot et quanti poetae [...] extiterunt" (Cicero, Tusculan Disputations IV,2/5) => "How many (quot) and how great (quanti) poets [...] have risen among us".
It comes from the neuter 2nd declensioncrustulum. The accusative form of it is crustula (neuter plural nominatives, accusatives, and vocatives tend to end in 'a').
Crustulam is the first declension accusative singular form of crustula (little rind, little shell, little crust).