That depends on what precisely you mean.
It's easy enough to give each site a number and each photo within a site. See the attached XLSX form for a working example.
However the way that Survey123 works, the actual output will have a separate record for each site, and the [photo number] field will always be simply the maximum number of images.
There are two options to achieve what you want.
The first is to use the Beta version, which allows you to choose the name images are stored with. If you use this option, the photos will be automatically saved as "Site 1 photo 1.jpg ", "Site 1 photo 2.jpg" and so forth. I've included this functionality in the example above, but if you aren't using the latest beta version, it won't work.
The other option requires you to put the photos in their own repeat - ie a repeat within a repeat. If you do that, then every photo will be saved as its own record with its own record number.
