The title of the question is bad so let me expand below.
So Prince Charles of the United Kingdom is the Duke of Cornwall, Andrew is the Duke of York, and so on and so forth.
When Charles dies his title of Duke of Cornwall will pass to William but Andrew's title will pass to his daughter (I think) or some other heir.
So my question is, if William has multiple children and becomes king, and needs to give them each a title, what does he do if there are no titles left to give?
There are titles which are traditionally associated with the royal family but titles are not in finite supply. The monarch can easily issue letters patent for any new title they wish at any time as the monarch acts as the fount of honour.
There has been an informal policy of so called economy in place whereby the British monarchy is making fewer royal peers than they have in the past so it's entirely possible that not all of William's children will be granted peerages. Assuming they are granted and none of the current Dukedoms have reverted to the crown then there is a long list of titles which have been historically granted to children of monarchs which could be granted again (e.g.: Clarence, Albany, Sussex, Cumberland) but it's just as possible that a new title might be created such as in the case of the Earl of Wessex or any other title that might be associated with the individual in question.