"Twenty mice are running behind the curtain."
Translation:Vingt souris courent derrière le rideau.
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".