If it makes you feel any better to know this, kids with siblings ask this. In fact I recently had my nearly 8 year inform me that he is ready for another sibling in about a year, for what reason, I don't know. My 10 year old love babies, so he has wanted one all along since my youngest was born, 3 years ago. My older two never wanted any. I adopted them, so I think they had fears that any birth children might displace them (it didn't, of course). My youngest isn't terribly verbal yet, but I am sure I will hear it from her sooner or later. I was the youngest of a large family who begged for a little brother or sister.
So all that to say, you will have to likely handle this question whatever amount of kids you have. ;)
I can't have more, so I tell them that. That doesn't stop them asking. I do not hide that at all, I tell them that the last pregnancy had some problems that likely would happen again for any future pregnancies, so I wasn't willing to try that ever again. I also tell them how happy we are with the size our family is right now and have no plans to make it larger. We haven't ruled out ever adopting again, older kids, so I also tell them that. We may at some time decide to adopt more, but not right now. I also tell them that if we are considering it more seriously, we will let them know that. We won't surprise them with it, or wait until last minute, but for now this is all there is.
I also tell them I understand wanting more. It's okay to want more people in your family, but that it is a decision that has to be up to the parents, because we have to make sure we feel like we can give enough to all the kids and still have enough room for ourselves.
It's an evolving conversation the older they get, the more they can understand. I would keep it simple and as honest as you can be. I currently feel like I can't really take on more at this time. Maybe I won't ever feel that I can take on more than I have. Maybe when these kids are little older, I will feel like I have more I can give. I really don't know, so I tell them that. I don't want to say "never any more" when I am not 100% sure. I also am not sure what the future holds, so I can't say it's going to happen. It may not.