It all depends. Do you have fish? If you do, are there any places on the surface of the pond that are completely still. If so, you need more. Do your fish hang out at the water return? If yes, you need more.
Fish require oxygen in the water to breath. If the oxygen is depleted, the fish simply smother.
You may have happy fish during the daylight hours with enough bubbles to go around - but it becomes a different world at night. Plants, who choose the night as their time to grow, rob the water of oxygen. So - DO NOT turn off your bubbles (your fountains or waterfall) at night. This is when it's needed most.
In mid summer in mid day temperatures soar and there's no relief for you. Heat, another oxygen theif, is working on your pond. The power goes out. Yes, you do need to worry. Get power to that pond as soon as possible or get bubbles into the water or you stand the chance of losing more than your temper. This has happened to us. We ran lines from a neighbor's house. Luckily we were the only house affected.
If you use chemicals to clear your water (or for any reason) watch your fish closely. Are they gasping at the top of the water or hanging around the water return where the bubbles are? If so - Increase aeration. Chemicals steal oxygen too. It's always a good idea to increase aeration if you're not sure.
The sign to watch for is your fish gasping for air.