Tony, that's good for them and a couple more but not too many. Stay with the smaller species and you'll be fine, teals and shovelers, bahama pintail, smaller docile stuff like that.
Mine were all 50 x 150 or 50 x 200. Those could hold a couple hundred easy.
I'd say 8-10 pair would do fine.
One thing I want to point out though.
Try to make it more than 8 feet wide and here's why. These type of ducks are captive raised, but are still pretty flighty at times. Especially mandarins and wood ducks, they are the worst of all of them. If you walk slowly threw the pen and dont act like you are pushing them, they just walk the other way and do problems arise. Problem with an 8 footer is you are too close to them at any given time that direction, they'll keep walking to the end til they feel trapped and start to freak out.
Just a suggestion, space may not permit it, but if it does, I'd try to double the width.
Pond size sounds perfect. a foot will do in depth BUT with species like mergansers, the added depth is a plus for them. If you arent familiar with merganser care, you need to read up a bit on them first, they are a bit different dietary wise than most ducks due to their fish diet in the wild.
I have a thread in the duck section about all of them if you havent seen it, I d advise reading threw the first 7-8 pages. It'll help you a lot if you are totally new to it.
It's the migratory and ornamental waterfowl thread and it located in the duck "sticky" section at the top of that page.
Good luck, you'll love them
