I think it's different for everyone. I think ALL of us get baby fever no matter how many kids we have. With few exceptions, I think we all wonder at times if we should have another. Being pregnant and having a little baby can be difficult, but it's truly wonderful, so it's natural to want to do it again. However, as a mom I know pointed out, wanting a newborn is different than wanting to have to raise and be responsible for another life for 18+ years! Another mom I know jokingly said she stopped having kids when she finally figured out they don't stay babies.
I read an interesting study about why people choose to have multiple kids or only one. The main reason is that they love their child. Either they love their child so much they want another one to love like that or they love their child so much they can't imagine loving another that much and/or want to be able to devote all their time and resources to that child (the average cost to raise a child from birth to 18 *NOT counting college* is almost $300,000, and it's surely more in this area). It just depends how you react to loving to your child(ren).
I think that solid agreement between both spouses is crucial to deciding to have kids. If either of you isn't totally sure about another, I don't think you should try. In my mind, that's just far too big a potential strain on your marriage, and you don't want to put that on the child. Oh, and my youngest brother is a surprise baby, so I've always though you should stop before you get to your limit because you never know but you might reach the limit without trying!
You might end up having another, but I think you should focus on being a mom to the kids you have. If both you and DH simultaneously start to feel like you want another, then you can cross that bridge. Oh, you might also want to think about foster parenting. You can foster infants -- it's not easy work, and especially with infants you have to be prepared to give them back, but opening your loving home to a child without one is God's work, for sure.