There is essentially no limit to the number of aquatic plants that can have a water garden (when you can no longer see the water because there are so many plants, it may be time to stop adding more!), but with fish, there sure is!
If there are too many fish, the oxygen level in the water will drop and your fish will suffer. Also, when there are too many fish, they produce more waste than the bacteria and plants can filter out and the water will become polluted and smelly.
To know how many fish your pond can hold, calculate about one goldfish per three to four square feet of water surface area and one koi for every 10 square feet of surface area. You need more space for koi, as they grow larger.