Construisez une bonne souriciére, et on se bousculera à votre porte.
why vingt does not has the plural s? and why the phrase does not start with des?
you have 20 "mice", but only one "twenty". So no S on a number.
a numeral before a noun replaces any article. Same as English. You don't say "the 20 mice". You just say "20 mice".
why le rideau and not du rideau