That is a good question. Answering it is surprisingly difficult. Every digital camera I have owned shows the approximate number of photos left on the LCD display on top. But both of them are Nikon and I learned the hard way all manufactures have different display designs. (Before LR I had built my own database for cataloging photos and renamed the files with the camera type, serial number, and shutter count from the EXIF data concatenated as a unique key. Then I found my friends Canon does not have shutter count in the EXIF.) But if your camera does than stop when you have a couple of hundred shots left.

In the first place the 64G does not mean you have 64GB available for your use. The file access table takes a modest amount of space but SD cards also contain DRM data designed to render the card useless should you tinker with it in attempt to copy the whatever you think you bought. That seems ti takr quite a large amount of space.

Second all files are not the same size do to compression. Even RAW files are lossless compressed and vary in size. So even if you know the available memory you have to do some division by say the max size. Windows Explorer tells you the space used and available. So Apple Finder must also. So you can see it there.