Vicky - yes and no.
Yes, most people do overstock until they learn better. Its because "just one more" little fish is so easy to justify, and they are so pretty.
But, no...The people who know better may have a small pond with a HUGE filtration unit. Part of the 1,000 gallons can be held back in the filters, which allows you to have a viewing pond - small pond so its easier to see and apprciate the fish near each other. Viewing ponds are heavily dependent on reliable pumping. If the pumps go out, the fish die.
Viewing ponds are what you see at professional aquariums and so on - because its more attractive to people who don't understand how hard it is to maintain.
If you are overstocked, you will have a crash. Usually its something natural that doesn't have room to be absorbed, like a small ammonia spike, or a big Ph change. Something like that, if you have more water, won't bother the fish much. But if you are overstocked - will kill them all very quickly.