So 18 months is the leeway between c-sections? Is that if you want a VBAC?
Yup if you are wanting a VBAC thats the recommended amount of time between births. Having said that I had 16 months between mine - so depends who your care provider is I suppose.

My sister has had 4 c-sections, the last two being a year apart exactly (her 3rd child was stillborn, and she fell pregnant with her second daughter 3 months later).