Impact crater density has been used to estimate relative ages of major Venus geologic units, typically based on the assumption of short, globally synchronous formation periods. The total number density of craters is the only parameter which may be quantified for most Venus subregions, in contrast to the size-frequency analysis often used for other planetary studies. This paper examines the statistical limits on such crater counts and the predicted crater density for various possible time-varying surface formation rate functions. These results show that (1) the true mean crater density is poorly constrained due to the limited sample size provided by aggregate geologic units, and (2) any given crater density can be accommodated by a very short pulse of unit formation at some age T, Gaussian-distributed formation functions of mean age <T, exponential functions with initiation ages between T and 2T, or a host of other more complicated rate functions. These results, when coupled with the unknown rate of crater removal by tectonic or volcanic activity, indicate that the crater counts for specific landforms on Venus offer little constraint on the relative timing or duration of their development. The hypothesis of globally synchronous formation of landforms such as volcanoes or coronae cannot be validated with the available crater data.