How many people do I need on my list?
Question - How many people do I need on my list to consider it a real business?
Answer - As for people on your list, you can never have too many :-)
The real answer is that it doesn't matter how many you have, you want to treat them well, build a relationship with them, and you can make a great living from a list of a few thousand.
I know a guy who made 2 million last year from a list of 8000, so it's all about the relationship.
What does that mean?
Give them great CONTENT and not just pitches for products. You should have one friendly pitch (in the form of a product review or the message I mentioned earlier REPEAT IT for every 3-5 pieces of useful content you send (I know I haven't always done this (my bad) but I am doing it now).
You can mention a product each time if you really want to but think about how you feel when you get pitched all the time. You want people to look forward to your messages, not delete them because they are too lazy to unsubscribe.
I have built lists for others (as partners) and they have done really well but for various reasons things fell apart. I have several lists of my own that do well, and I'm building more all the time, so yes you can do well if you focus on one niche (your photography niche is great) and build that relationship.