"It depends" is a lame answer, but it definitely does apply here. Being prescriptive about the correct coffee-to-water ratio is impossible (and anyone telling you they know the answer is just making it up!). It's highly dependent on your ingredients (beans, how they're ground, roasting method, etc.), the brew method (time, fridge vs room temp, type of water, etc.) and most importantly your individual taste preference. So if you Google it, you'll see an incredibly wide array of opinions about the correct ratio.
So our best advice is to use your Rumble Jar or Rumble Go to figure it out yourself. We've included those notches (i.e. short horizontal lines) on the side of the the metal filter as guides, not as prescriptive tools. They're there to help you eyeball your grounds as you prepare a batch and roughly figure out how much you personally prefer. We've found that the vast majority of cold brew drinkers prefer to fill up the filter with grounds somewhere in between the lower and upper notches.
Rumble Jar: The lower notch is our best guess for where you'll want to start (~1.5 ounces of ground coffee in the quart size and ~3 ounces in the half gallon size). Fill your grounds to that line, drink a batch of cold brew and then reassess to figure out whether you want more grounds for your next batch. The higher notch holds ~3 ounces of grounds in the quart size and ~5 ounces in the half gallon size. Some folks like to brew a higher-strength "concentrate" and then dilute it with water/milk/ice when they drink it, so that's an option too if you want to get more cups out of a single batch of cold brew.
Rumble Go: Given the height-adjustability of the base, the capacity of the filter changes depending on how tall you make your filter. In general, we've found folks brewing in 16-24oz bottles fill the filter with grounds between the lower and middle notch. Folks brewing in 25-32oz bottles fill the filter between the middle and upper notch.