Esa Ruoho has a 11,000-item iPhoto library and an Apple Photos library for OS X that comprises over 6,000 items:
I just spent 6 hours combining three iPhoto Libraries (one on a MacBook Pro, one on an iMac and one on another iMac) via File Sharing between them, and the Fat Cat Software iPhoto Library Manager. So far, so good.
However, since the goal is to push 13,000+ images to iCloud Photos, I’m not sure how to either combine an iPhoto Library and a Photos Library, or how to combine the iPhoto Library imported to a Photos Library, and a Photos Library.
The good news is that this can be done. The bad news is that it takes more effort.
Read more here:: How to merge large iPhoto and Photos libraries