If you're looking to make a 5 gallon batch of corn mash, you'll need to start with around 10 pounds of corn. This will give you the ability to adjust the final volume and gravity of your mash to your liking.
Corn is a starch-based grain, and as such, it has a high potential alcohol yield. This means that, in general, you'll need less corn to make the same amount of mash as you would with other grains like wheat or barley.
That said, the amount of corn you'll need to use will also depend on the method you're using to make your mash. If you're doing a traditional infusion mash, you'll need to use a larger amount of corn to account for the water you'll be adding.
If you're doing a no-sparge mash, on the other hand, you can get away with using less corn since you won't be losing any of your wort to runoff.
Ultimately, the best way to determine how much corn you need for your 5 gallon batch of mash is to experiment and find the ratio that works best for you. Start with 10 pounds of corn and adjust from there until you find the perfect balance for your brew.