How do I find out the dpi and pixels of a photo in my library?

Select any picture and the right-click/two-finger click to select "get info." Or use command+iYou'll see the size of the image in pixels.

For dpi, you would need to open in Preview (or other application). right-click/two-finger click then select edit with, Preview. Then use command+i

select the photo from your Photos app, & click on the "i" inside a circle at the top right

The dpi depends on the size of the photo you plan to print. The dpi will change for each different print size selected. Some 3rd party apps like Photoshop Elements for Mac will let you put in the print size and dpi desired and will change the pixel dimensions to match that.

See this website: The Myth of DPI.