Representative sample size calculation.

I want to manually analyze bug reports of three large software projects. Total bug reports of the three projects are 10,000, 12,000, and 8000. I need to examine bug reports, comments, and bug fixing files. Manually analyze all bug reports are a time-consuming and difficult task. For these reasons, I would like to take a sample of bug reports from each project. Would you please suggest me how many bugs reports from each project should I analyze to make a representative sample size.