It can't really be answered.. But I don't know if you're asking for merchandise sales or the exact number of fans which is almost impossible to answer.
But there are some college football teams like Ohio State, Michigan, and Notre Dame who have more fans than any NFL team out there.
But for individual fan base the teams are like this for merchandise sales yearly average.
#1: Ohio State, has the most fans (Estimated) and also rated the most trash talking fans alive.
#2: Michigan, this is another reason why they are rivals because of their huge fan base, Michigan and Ohio State by themselves have more sales of the entire SEC and Big East conference combined.
#3: Notre Dame
These are the most money making teams out there, no other NCAA football team makes anywhere near as much as these three teams. Ohio State makes over $8.9 Billion dollars a year in sales, Michigan with $8.1 Billion dollars, Notre Dame with $7.6 Billion.
Fourth is USC with $2 Billion.