Why did the anime have so many additional cards? In the manga, there are 19 cards featured, whereas in the anime this number has grown to be 52 (excluding the card created during the 2nd movie).

Has there ever been a reason given for this large expansion?
As far as I can tell, there has never been an explanation given for this change. However, there is one major difference between the manga and the anime that may have led to this. In the anime, Meiling Li comes to help Syaoran Li collect the Clow Cards. This may have caused a need for more episodes to deal with her getting in the way of Sakura collecting Clow Cards, which required more Clow Cards to exist.

This is the major difference between the manga and the anime in terms of the collecting Clow Cards arc,  but this is only speculation. 

There are six "leader" cards (The Dark, The Earthy, The Firey, The Windy, The Light, and The Watery) and all remaining cards, whether part of the original 19 or part of other 33, fall under the leadership of one of those six. While the creators of the anime made 33 more cards, they did not make any more leader cards, implying that these cards are filler.

More information about the Clow Cards can be found here: http://ccs.wikia.com/wiki/Clow_Cards