When I take photos on my IphoneX, I always upload them to my mac photos.
How do you upload? Do you connect the iPhone to a USB port and launch Photos?
If only some Photos appeared in Photos, check, if iCloud Photo Library has become enabled on the iPhone and "Optimize Storage".
Check "Settings > Your Name > iCloud > Photos". Is iCloud Photo Library enabled,mand "Optimize Storage"?
Then you may not be able to download all photos, because some may only be in iCloud and not on your iPhone.
iMac, OS X Mavericks (10.9)
Is your signature still valid? Or did you upgrade to a newer system?
If your iPhone X is set to take photos in the space saving High Efficiency format, a mac with Mavericks is not compatible. Then you need to change the settings on the iPhone.
See this document how to set the iPhone to be more compatible with older Macs: Using HEIF or HEVC media on Apple devices - Apple Support