There are roughly 4 cups in a pound of typical dry dog food and 3.75 lbs of dog food in a gallon. However, cups and gallons are based on the food volume, which is a bit different for each brand of kibble.
Some dry dog food brands like “Royal Canin” seem to have about 5 cups in a pound, and others seem to have about 3 - which averages to about 4 cups.
To get the exact answer for your specific dog food brand, you’ll have to measure out the dog food yourself, using your own cup. Put a bowl on a scale and start counting the number of dog food cups until you hit the 1-pound mark on the scale.
Please note that these are approximate numbers as each brand of kibble will be slightly different in volume.
Check the guide printed physically on the bag to know how many cups of dog food you need to feed your dog per day. It will usually contain a simple table you can follow based on your dog’s breed, weight, or age.