"I am buying two spiders."
Translation:Kupuję dwa pająki.
it is never "pająka", after 2,3,4 you treat plurals like there is no numeral in front of them. kupuję pająki, kupuję dwa pająki. dwa is also declined,
after 5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21 , and most quantifiers you have declined form of number and noun in PLURAL genitive
Or to put it in other words:
If the number end in digit 0, 1, 5-9, or if it ends with numbers from 10 to 19, you would use genitive plural (as well as with quantifiers; like „żaden”, „wiele”, etc.). With other numbers (except one, of course) you use nominative plural.