This is a very common question, and the answer -- in my opinion -- is quite simple: wait until you are done having children.
You can become pregnant after a tummy tuck, and it will not adversely affect your child, but you will undo everything you have achieved with the tummy tuck. So you will have wasted your time, money and recovery on the operation. And having another tummy tuck after the pregnancy, may not give you as good a result because of how the pregnancy will expand the already contoured abdominal wall.
If you are not sure about more children, wait on the tummy tuck until you are. The tummy tuck is an elective operation -- there's no rush to have this done (from a medical standpoint). So, wait until you know for sure. I'm sure your plasitc surgeon would give you this same advice.