I own a Giant Revel 29" 2015 which has 2.1" wide tires. I want to go wider.
What's the maximum size of tire that I can put on these rims?
The Giant bike specs page says nothing about tire clearance. You can measure it yourself:
For example, if current tire is 53mm wide and clearance at each point is 1.5cm, about 1cm wider tire might fit (about 2.3"). Keep in mind that tire width will vary from claimed depending on manufacturer and your rims inner width. You should also factor in riding conditions, especially amount of mud. And make sure that new tire will fit into rim brakes, if that's what the bike has.
You might find it easier to visit a local bike shop and find a suitable wider tire together. Just fit something bigger and see if clearance is good enough.