Baby Number 2?

So I’m 18 currently 34 weeks pregnant with my first child, a little girl. Her father and I are separated as we are on and off a lot. We were separated when we conceived her. Both of us want to try for a son once she is 6 months old, which will be around January 2019 as we were having sex for 3/4 months when she was conceived we figure she’ll be about a year and a half when our second is born. But some people tell me that’s too soon to have another one, opinions?