Depending on the niche and your followers 1-5 photos per day.
I've got accounts that only post 1 photo and get more engagement than others that post 4. On the other hand, I have other accounts that post 4 times per day and get more total engagement than all others. Same niche on all for the above examples.
What does that show you? In my experience, it's a percentage of users, times, divided by the photo subject, multiplied by the day of the week, and then the square root of the total.
Whatever works for me, or anyone else, may or may not work for you. Test it out with one photo a day, then multiple photos, then try different posting times until you find YOUR sweet spot.
Hope this helps.