The Disney photographer will take much more than just 20 pics (usually closer to 70-80).
The thing with the package is you only get 20 in the album that is included.
You have a couple of options about this:
The first one (and the one I did, and recommend, if you decide to use Disney Photography) is to PRE-purchase your proofs. It will cost you about $250 up front, but it's worth it. You can also wait three years and purchase the Hi-Res CD from Disney for about $100 (check the price, it may have gone up) and then you can make all the reprints you want, where ever you want.
The second option is to purchase the Hi-Res CD at the time of your wedding, at a cost of about $1000. Then you can reprint all your images.
Many brides consider the outside vendor to be more cost effective. I did not use an outside vendor because it's an additional cost, over and above what is included in Disney's price, and I was on a very strict budget. But, for many, the expense was well worth it! Randy Chapman comes highly recommended, as do a few others.
When all is said and done, I will have paid about $300 for my wedding pictures, and that is reasonable to me. Yes, I have to wait three years, but I know my pictures are safe (Disney isn't going out of business anytime soon) and I am patient!

And Disney did a fantastic job for me!
Of course, there are other factors. With Disney, you cannot pick a photographer. You can make a list of specific pictures you want, but there is no guarantee that the photographers style will match yours, or what you desire for your wedding. I think Disney tries to accommodate this, if you tell them up front, but, they just can't guarantee a specific photographer.
Hope that helps!!