random sampling query

Hi all

I need to look through medical charts for my project. The medical records department has informed me that that there are 5000 charts that meet the criteria for my project. But I haven't got the time to look through all 5000 charts.

So I'm intending look through a random sample of these charts to ensure that they are do indeed meet the inclusion criteria as there is room for human error. Any idea on what proportion of the 5000 charts I should randomly select for this? Do you know of any stats books or articles I can quote for using this proportion?

Thanks heaps!!!