I need to clarify what some of the ladies above suggested. I'm Rh-, married to my Dh who is Rh+. Our DS is Rh-. I only had 1 Rhogam shot with him (around 28 weeks), as I didn't need it once DS born since he too was Rh-.
Blood types have two Rh factors in the case of Rh+ dominant carriers. There's the recessive factor, which may be another + or a -. My parents are both Rh+ and I'm a product of their recessive factors. If you're Rh-, all you have on dominant and recessive are -s. So this makes my DS a product of DH's recessive and all I can throw.
So being Rh-, you may only need one Rhogam shot, depending on what LO's blood type turns out.