How many gels is the right amount of gels?

Hey everyone!

I'm gearing up for the Miami marathon later in the month. First marathon and I'm getting super excited!

My question is - how many gels is the right amount of gels?

I've been experimenting with Gu's and Cliff Bloks but haven't really been locked into the my gel in take. I had my longest long run last week and I def didn't take enough gels during the run and felt less energy towards the end of the 20 miles (still finished somehow lol). 

Any advice from ppl in endorphins would be so awesome!!!

Thanks!