Once you have trouble reading your own code that you wrote some time ago, this is definitely too much.
I recommend programming in object-oriented PHP and using autoloaders to avoid having to enable / require as much as possible. Excessive use of include / require often results in unreadable and irreplaceable spaghetti code, which is very bad.
In small projects, I usually have one request to perform the autoloader function, and in large applications I use the Zend Framework, where I rely solely on Zend_Loader.
From a purist point of view, I would say: more than 3 includes / requires too much in your own code (without third-party libraries):
- One to incorporate some iniitialization material
- One to load autoloader class / function
- , . , / . .
.