Окно is neuter. This is the dictionary/nominative singular form of "window."
When counting two of a window, in Russian, you literally say, "two of a window" (see what I did there? ;) ) and when you use "of a," you use the genitive singular form of window.
In the genitive, anything ending in o changes its ending to a. So одно окно (one window), два окна (two of a window/two windows).
(Use the genitive singular from 2-4; 5 and up will use genitive plural - "5 of cats/5 cats, 7 of cranberries/7 cranberries," etc. and will have different endings in Russian.)