1. That is called a chisaron in emunah, not kefirah (if a person works eighty hour weeks because he this that how much he works matters, is he a kofer?
2. After a person discharges his biblical obligation of a boy and a girl, he now has a chiyuv midivrei sofrim of la'erev al tenach yadecha. If he is in a situation where it becomes irresponsible to have more children, as determined by a rav (see my example above) I do believe he can adversely effect his economic situation by having more children. Does that make me a kofer?
3. It used to take a lot to be a kofer. Apparently nowadays every Tom, Dick and Harry is one.chazal have say if someone call someone else a rasha he can yored imo lchayav-all the more so if you call him a kofer. I would tread carefully