What happens if i delete photo copies on my mac?

If you are synced with I cloud - then yes, they will disappear also from I cloud. If you want to reduce space used by the photos on your mac, you can either:

1 - Move them (the whole library) to an external drive.

2 - turn on "optimised storage" on your mac - then the full resolution images will be stored in I cloud, and smaller copies stored locally. But that will apply to ALL your images.

Turning on optimisation is the right approach, but Photos will only start replacing the high res files with thumbnails when your computer starts to run out of space. It will not just optimise the whole library the moment you switch it on.

The best thing to do is to create a new library, set to optimise and then Photos will only download the thumbnails. However, if you edit a photo or export a file then Photos will download the full high res file (to allow you to edit or export) but then the high res file will stay in your library. Therefore, don't be surprised if over time as you use it, the library size starts to grow even if you don't add any new photos to it.

Also, remember iCloud is a syncing service, not a backup service. Its purpose is to keep your photos and edits synced across all devices. You should make sure you have a backup somewhere of all your photos. Whilst iCloud is very reliable, there are stories on here of people setting up new devices or things going wrong and they lose all or part of their library and once its gone there is no way of recovering them. (Yes deleted photos are kept in a folder for 30 days, but there was a question on here just a few days ago where someone didn't realise a chunk of their photos had been deleted for a few months and it was too late to do anything - remember, BACK-UP, BACK-UP, BACK-UP!! My approach is that I have an 'optimised' library on my laptop as storage space is low. I have a 'download' originals on my iMac and I back this up periodically to 2 separate drives as well as Time Machine. Every so often I check the number of photos and the number of edited photos shown in Photos to make sure they are roughly the number I was expecting (about 2 years ago the number of edited photos fell from 4,000 to 1,000 literally overnight- I have no idea how this happened, Apple Support couldn't help, but I was able to recover all the edits and the hundreds of hours it took, from one of my back-ups.

The optimisation will only remove the high resolution original photos, if storage is needed for other applications. It will take a long time for the library to shrink. But that is the expected behaviour. Photos is reluctant to delete downloaded photos needlessly, as long as there is some free storage on your Mac. Usually the optimisation will kick in, if your free storage is dropping below 20GB, but not before. Otherwise the recently downloaded photos will just be kept around, so you do not need to download them again over and over again. Why waste the free storage on your Mac?

The purpose of iCloud Photos is primarily a syncing service. It is perfect for keeping your Photos Libraries identical on all your devices. The library is kept in iCloud as the Master Copy, and the local libraries on your other devices are mirroring the library on iCloud. If work with your library on one device, all changes will be updated in iCloud and all other synced devices. Delete a photo on your Mac and it will be deleted in iCloud and your iPhone, or whatever you are syncing with iCloud. iCloud Photos is not a separate external storage - consider it as the primary central working copy of your Photos Library. When you are working with your photos, you are modifying the iCloud versions of the photos. The bonus is, that this central copy is stored off-site. If your house burns down, you can recover your photos from iCloud. But you cannot recover photos from iCloud, that you accidentally delete permanently. It is not a backup.

Usually your Time Machine backup will suffice as a backup of your Photos, but not, once you enable optimise Mac Storage. Time Machine cannot backup the optimised versions, that ar only in iCloud. You have to make a separate local backup: How to back up an optimized iCloud Photos Library - Apple Community

See also: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community