"Il a de grands chats."
Since there is usually an article before a noun in french, you should pay attention to the article before it to differentiate if its a singular or plural noun. When a noun is singular, it would always have one of these article before it: un(for masculin noun), une (for feminin noun), le (m), la (f), l'(nouns that start with a vowel). If the noun is plural then: les (mf), du (m)(the "de" series can mean "some" [although not really plural but if otherwise I dunno how to categorize it XD]; shortened for "de le"), de la (f), des (m), de l'(vowel), and also in this case just "de" because there's an adjective before the noun. Note: de can also be used for plural nouns for phrases such as: beacoup de, plus de, pas de......etc. Sourse: highschool French
If you want to look up the grammar online, look for "partitive" (which is the combination of de+article) and "adjectives".
So in this case, the rule is: 1) use the partitiv (de la/du/des) in front of "adjectiv + singular noun", for example Nous avons mangé du bon fromage. Nous avons écouté de la bonne musique.
2) use de (instead of des) in front of "adjectiv + plural noun", for example On l'a constaté de nombreuses fois: cet homme donne toujours de précieux conseils.
However, in spoken language, people often use "des" but only for short and well known adjectives such as beau, bon, faux, joli, grand, petit, vieux.
Plus, if the "adjectiv+plural noun" form a fixed expression, then there is only des: des jeunes gens, des jeunes filles, des grands magasins, des libres penseurs
Source: Klein/Kleineidam, 1996
clarasmiler, Though my hint seems rather "elementary", you could watch the "s". 'Grand' (male singular); "grands" (male plural). Moreover, for female nouns and their plural forms, you add an "e"/"es" ("grande"/"grandes"). Generally, I think these rules hold. For instance, (big cat) "Grand chat"/"grands chats"; (big skirt) "grande jupe"/"grandes jupes". Please, comment if you find some mistake. Good luck!