Translation:Fifteen cooks are cooking your lunch.
It's more like "15 is cooking your lunch". "cooks" are just an "extra word" to understand it better ;)
What can I say, numbers are complicated.
OK, so "1" obviously takes singular, but then you have the following numbers: 2, 3, 4 and those ending with -2, -3, -4 BUT NOT -12, -13, -14. All those numbers take nouns in Nominative and they take verbs in plural. So they are the 'logical ones'.
The other numbers take nouns in Genitive and verbs in singular.
Seems complicated? Wait, there's more! Even those 'logical' numbers have separate forms for the 'masculine personal plural', and those forms take Genitive and singular verbs as well! So you have "Cztery kucharki gotują" (four female cooks are cooking), but "Czterech kucharzy gotuje" (four male cooks are cooking).
Seriously, numbers are in the top of the most complicated things about Polish. With bigger numbers in some cases even the natives get confused a lot.
Technically, a "chef" isn't a "cook", but as its French original meaning implies, (s)he is the "chief(=boss) of the kitchen", although (s)he might well do some cooking. Polish for a "chef" seems to be a "szef kuchni".
I would have expected kucharz to be in the genitive plural. What are the forms here? Are there tops and notes sections I am missing?
You may find some rules for numerals here
http://www.placedauphine.net/random/PolishGrammar.pdf page 47
These links work in the app but most others don't. Probably because they are just naked URL's and not entered as an HTML-tag. It could be a good idea to always write links like this for the benefit of app users.
How does one access these sections in general? I have been wondering why there where so little explanations.
Well... if we accept "you all" generally, then there's no reason to reject this...added now.
y'all's is a regional dialect of the southern United States, and is perhaps used by Urban African-Americans as well but it is not Standard English. it would be like saying "ja młuwim po polsku"