I want to buy the ipad Air but I am unsure roughly how many average quality photos 16gb will hold please?

If your mean 64GB the answer is a whole lot.

At the time when I sold my 16GB 4S, I had over 4,000 pictures in it plus about 700 songs.

16GB vs 32GB vs 64GB: Which new iPad storage capacity should you get?

http://www.imore.com/2012/03/08/16gb-32gb-64gb-ipad-capacity/

How much content will fit on my iPod or iPhone?

http://support.apple.com/kb/HT1867

How Much iPad Memory Do You Need?

http://ipad.about.com/od/iPad-Tutorials/a/How-Much-iPad-Memory-Do-You-Need.htm

 Cheers, Tom 😉

I have a 16GB iPad, I would recommend a 32GB to be on the safe side. You can't buy additional memory afterwards.