Can I use more than one EX6120 with my network?

I don't generally recommend more than 1 extender. 

when users are needing more than 1 extender, I usually recommend moving to an actual mesh system designed for multiple nodes. 

With that out of the way, You can use more than 1 extender. 

I'd highly advise you against daisy chaining them. I'd set them up in a star configuration.

Star: Extender<--------router-------->extender

Daisy: Router------->extender------->extender

Reason why is the extenders have to use the same chip to go route---->extender and then extender---->devices. And they can't do both at once. so their thoroughput is cut in half.  If you're daisy chained and add another extender on the end and its cut in half again. so the 2nd extender at best would only get 25% of the original speed. And thats without taking into account interference, obstructions, distance, etc. With those included, your speeds are significantly lower.  Plus it increases latency. 

so overall, can you do it? Sure. 

Should you do it? Depends on your situation.