I'm not so sure it really depends on the number of pages you are going to server; rather, the number of users/changes you expect. There are some sites I have built with about 30 pages and the sections that contained the most data that were changed, needed to be searched and added to often, I went with mysql. Less accessed/less changed pages, I went flat file.
Let me give you an example.
I did a site for a photographer. His stock photo collection, and an oline portfolio were done with mysql, his biography and Meta Tags were done with a flat file. Can you see the difference?
On the other hand, I did a search engine (very small) that has a 7 meg db and gets about 1000 users per day (growing) just about everything is using the db just for efficiency.
Hope that helps.
