Depends on your personal preferences and what your body can handle. I find I get stomach problems if I take in too many cheap sugars, so at a marathon I did earlier this year I think I had three altogether, and didn't take my first until 20km or so. It worked for me (no cramps or anything) but what works for you will take experimentation.

I have a couple of gels, a couple of shot bloks, gatorade, and whatever food they offer on the course, bananas oranges, pretzels, whatever.. ;-)

I think there is no magic number of how many gels or equivalent is correct for a marathon distance event.
As the rest of the guys said, it will depend primary on how your stomach can handle the concentrated sugar and lytes that come on each gel.
Some people, lets say like, Lance Arstrong I read that will have one gel every 15 or 20 min, or even more, and nothing happens with his stomach.
Some other people instead might get huge cramps, diarrea, nausea and vomiting, by taking too many and too often.
So the best advice, experiment on your training runs; also very important, you might need to try diferent brands, until you find the one that your body can handle the best, with or without caffeine, and so on....

My personal number, 4 or 5 during a Marathon distance....plus Gatorade....