iCloud Photos

I am getting the iPhone 11 today. I was trying to transfer all of my iPhone 10 pictures over to my computer. I Turned the iCloud option on and in the process all 48,000 photos I had on my computer went onto my iPhone 10. How can I transfer all of my pictures onto my computer without having to put more on my iPhone 10? And when I get the iPhone 11, I don't want all of those 48,000 pictures on there, only new ones. Any help will be greatly appreciated!