I have a 16GB 512 SSD MacBook Pro and am wondering how many photos this will hold?

Depends on the size of the images. It will hold 512GB. Some of that will be used by the system and other data. It is impossible for us to say how many photos it will hold because we don't know the size of each photo. Total storage is 512GB. You have to leave at least 15% free for operations such as swap and temp storage.

Any computer typically holds about 100 fewer photos than you want to store, so you will need an additional drive.

While you are shopping, remember that your computer drives are not permanent storage, and you MUST create a backup on another drive -- otherwise you risk having a relatively minor computer problem wipe out all your photos.