In the most common sized foam cannon container, which is 33 fluid ounces, you should use between 2 and 4 ounces of soap, with the rest of the bottle filled with warm water. Though you can use more soap if you are dealing with something like a particularly nasty stain, or you can use less if you are using a detergent.
This is just a general guideline; however, so you should always read what the manufacturer of your product recommends as a dilution ratio, this will be found on the bottle itself. Some foam cannon foams will be more concentrated while others less so. Though there is also the chance that you may be using a pre-diluted formula that you would just pour straight into your bottle without adding any additional water.