I would like a third in an ideal world, but we have stopped at two and I am OK with it.
Reasons:
DH is 46 and already feels like 'an old dad'. He doesn't want to be changing nappies into his fifties or be too old to run around the park with them.
Finances - would impact on everything. At the moment we have a comfortable life, but another child would definitely put a financial strain on us.
My health - I don't 'do' pregnancy well, was very ill in my second pregnancies, and have had two c-sections and would definitely need another, which means recovery time.
As my two children get older, I realsie that the baby stage, as cute as it is, is just one short stage. And as my two get older, they need more investment of time and energy in some ways. Ok, so they aren't completely dependent on you anymore, but their education needs proper care and attention, their social lives need managing (bloody after school clubs etc!), and they become proper people, with minds and wills of their own. Two is hard work. Three might finish me off.
In all honesty, I would probably ignore all of this and have a third child tomorrow, but I know in my heart of hearts it makes sense to stop now. Quit while you;re ahead, so to speak!