Can I take a 300 dpi photo with my iPhoneX?

Here is a user tip that shows how to change the dpi tags with exiftool: Quick Action: How to change the dpi tags with exiftool

It also describes how to embed the exiftool command into a Finder quick action (needs Mojave).

Your iPhone X back cameras retaking photos at 12 megapixel. The standard size will be 4032 × 3024 pixel.

If you print these photos at a size of 13 inches at the longest edge, they will have a resolution of at least 300dpi.

The dpi (dots per inch) are determined by the size of the printed photo, not by the device that is taking the photos.

The iPhone is setting the dpi tag 72 dpi, but this is just a hint, how large the photo can be displayed or printed.What really matters are the number of pixels and the size on the paper, when you print it. As long as you do not print the photo larger than 13.44 inches at the longest edge, you can print the photo at 300 dpi.

The iPhone Camera.app does not have a setting to change the default dpi embedded in the image file.

When I look at the files taken by my iPhone with exiftool, I can see, that these tags have been set. You could use exiftool on the Mac to modify the tags, but I would not bother.

Orientation : Rotate 180

X Resolution : 72

Y Resolution : 72

Resolution Unit : inches