In America the company's don't have to list percentage but ingredients are listed in order of the percentages. There are 9 ingredients before you get to the vitamins and additives that are likely less then 10% of the food combined. So corn and soybean meal being listed as the first two likely means they make up more than 25% of the food but I'd guess more like 25% each. You may be able to call the company and get a straight answer.
But I don't like Meow Mix so I may be over estimating that. Soybean meal I group with corn because it is a cheap plant based protein additive. Likely added to up their protein to an acceptable level to be recognized as a complete cat food. However, cats don't process plant based proteins well, if at all, so it is artificial. Plus all those artificial colors? By products in dry food? Animal digest? I really have a problem with it. It's just bad, my girl was fed Meow Mix before I got her and she was barely over a pound at four months old, her coat was sparse and course, and her poop was like orange chalk. Whatever money is saved feeding Meow Mix is countered by the amount of food the cat has to eat to feel and stay full honestly if you are looking at it as an option.
There’s no way to know for sure without contacting the company but given that it’s the first ingredient you can be sure that there’s more corn in it than anything else.
Don't just say "See it is bad!" tell them why. Tell them cats are strict carnivores. They do not need corn, vegetables and grains and really anything but meat. Improper diet can lead to many illnesses and obesity. Food like this isn't made of quality, filling ingredients and they cat will just eat and eat as their nutritional needs are not being met, getting fat, and obesity leads to diabetes. Diabetes is EXPENSIVE to treat fyi. Just no reason to feed them this terrible type of dry food. Really a cat doesn't need dry at all so try to talk them into buying your cats canned. Friskies Classic and Fancy Feast Classics even are very low carb foods and tend to be more affordable. Your cat will thank you in the long run.
Personally, I don't trust the cheap supermarket brands like Meow Mix and 9 Lives. You'd be paying for grain fillers they can't digest. Grain-free food costs more, but about the same amount of nutrition is absorbed in a smaller amount of food. Plus the cats generally like them better. It's a win-win.
And I have no problem with adding a little dry food to their diet, as long as it's the good stuff and they have access to clean water at all times. A few tablespoons of high quality dry food per day in addition to the can of wet adds almost nothing in cost.