sample size for a probit analysis

Hi there,

I am conducting a hydrocarbon toxicity research using mosquito larvae as target. The idea of this research is to estimate the relative toxicity of a specific organic chemical by exposing mosquito larvae to its aqueous solution and monitor the death rate. I am no statistic expert so I would like to ask what's the minimum sample size required to generate statistically meaningful data. Like 20, 50, or 100? And why?

Thanks in advance