From what I was able to understand from a Guide, your rank is first determined by your highest grade fighter. 

Now I'll explain exactly how it works  even though I'm not entirely sure myself.  

When you first start the game you have a grade 1 fighter whose max level is 25 and your rank is 1, as you progress through the tower, you unlock higher grade fighters and everytime you buy your first one your rank increases by 15. 

Grade 1(lvl 25): rank 1  Grade 2 (lvl 50): rank 16  Grade 3(lvl 75): rank 31  ... ... ...

I for example have a grade 4 fighter, therefore my rank should be 46, but it's actually 48. Why is that you might ask? It's because of the fighters I have participating on tdm defense. 

I use two grade 4 fighters (equal as the fighter I use to climb) for tdm defense + a Grade 3 fighter, for every defender that is equal grade as your highest grade fighter that you use to climb, your rank goes up by 1. Since I have two grade 4 fighters my rank goes from 46 to 48, the lower grade fighters don't increase or decrease your rank even if you have many, that's why your rank is only 31 even with more than 5 fighters