I wanted to know if the 5% sample IMPUS USA - CPS sample is nationally representative county-wise?
I am not entirely certain what IPUMS project or set of samples you are referring to. I’ll provide information for both IPUMS USA and IPUMS CPS. The general answer, for both IPUMS USA and IPUMS CPS, is yes the samples are representative at the county level for counties that are identifiable in the data. Note that not all counties are identifiable in either IPUMS USA or IPUMS CPS. See this blog post for more information on this detail. Finally, the sample size of samples available via IPUMS USA are typically large enough to provide relatively accurate estimates at the county level. This is not necessarily the case for sub-state level estimates using IPUMS CPS data, due to the smaller sample size.
The sample I am talking about is IPUMS - USA 5% sample of year 2000