"I would like ten cookies."
Translation:Decem crustula velim.
You're correct, later Roman historians said they used to only have 10 months and then an unorganized "winter". However there's some debate among scholars whether this legendary 10 month calendar was ever followed: https://en.wikipedia.org/wiki/Roman_calendar#Legendary_10_month_calendar
Moopish is correct. Latin's 2nd, 3rd, and 4th declensions all follow different endings if the noun is neuter. I suggest memorizing these charts (which may take time) on Dickinson educational pages. You can Google _(nd, rd, th, st) declension stem, paradigm, and gender, go to images, and you'll find extremely helpful charts!