I don't know about number. But I've had no problem dating as a single mom with 1 kid. My daughter's father had 1 from a previous relationship. And here is what I've learned:
For me, it is not so much whether or not the person HAS kids, I think the bigger thing I would look at is why he isn't with the mom. If he walked out on the wife, then I wouldn't date him. I'd also look at the dynamics with his ex-if it is high cost flict, no thanks-and if he is current on child support and if he uses his visitation to the max. And at what kind of a father he is.
Dating a single parent can be an advantage actually. You can see a lot about their character.
i wouldn't at this point... or probably ever, but if i had to choose, between 2 and 3. why? because it's young enough where they won't remember life without you, but old enough to be potty trained so you don't have to change their diapers and shit