I think too many, ultimately, is an individual thing based more on the amount of care you can provide and what you are comfortable with, and your neighbors and the personalities of the birds involved. I don't like hard and fast 'calculator' type answers to these questions because it varies.
However, I am going to tell you now:
Ducks smell - a lot. They are messy - a lot. Ducks take up a lot of space.
Right now I have *one* chicken, but I've had both ducks and chickens in the past, and as much as I *loved* our ducks (this was 10 years ago, in Michigan, with lakes - and spoiled ducks who went on walks on harnesses with us) I can tell you that 2 ducks smell more, poop more, and make a bigger mess than a dozen chickens. Kiddie pools are going to need to be emptied, cleaned and refilled every single day - at least once a day, and that makes its own kind of mess. You are more likely to upset your neighbors with ducks than hens, based on smell and MUD alone.
So, um. I'd think real carefully about that one, were I you.