The situation is kind of complex (but not too complex!). You're right in that two kittens is dà phiseag, and three kittens would be trì piseagan. But "phiseag" is also used for "one kitten" - aon phiseag. Piseag on its own means "a kitten". Piseag is lenited to phiseag when it follows either aon or dà. So while phiseag is used when you talk about two kittens, it doesn't on its own mean two kittens - it needs to be preceded by aon or dà to make sense.
Gaelic has a dual number that is used when we refer to two of something - as well as the singular and plural we're used to in English (so far Duolingo is thoughtfully restricting exposure to the dual to situations where the noun is the same in the singular and dual, and avoiding the use of adjectives in the dual). There is not much left of the dual number in modern Gaelic, and it is only used after the word dà . So if you want to refer specifically to "the two kittens" you would use "an dà phiseag" (note that you use "an" rather than "na"). But if you just wanted to refer to "the kittens" and there happened to be only two of them, you would use the normal plural "na piseagan" because you are not including the word dà in the phrase. So piseagan can be used for any number of cats above one, providing you don't use the word dà
piseag is the singular and piseagan is the plural
There's a lot explained in the tips and notes for each skill and you'll understand if you read them. If you have trouble accessing these from the individual skills because you're on a phone or something, the whole lot can be found on one page here. https://duome.eu/tips/en/gd