Will I loose photos by syncing to iCloud the first time ? (First time setting up)

Hi Guys !

Im setting up iCloud for the first time for pictures. My pics on my Mac go back way longer than on my phone. So my question is, if I set up an iCloud from my iPhone and sync my Photos app, will it only sync my phone and delete pictures from my Mac which don´t match the pictures on my phone ? or will it merge all the Pics and a combination of the pics on both devices will now appear on both ?

Thank you !