"There are two thousand residents on the island."
Translation:Il y a deux mille habitants sur l'île.
"Centaine" is a special case because it is not a precise number. It means "about a hundred" or "a hundred or so". When a noun follows centaine you must have the preposition de/d'. Please also note that it is always "une centaine", or feminine, no matter what follows it.
une centaine d'hommes (about a hundred men)
une centaine de biscuits (a hundred or so cookies/biscuits)
You'll see this in other approximate numbers with the ending -aine like: vingtaine (about twenty), trentaine (about thirty), quarantaine (about forty), etc.