M.O.
Hi N..
I think one or two letters would be nice. Other kids will probably be getting letters from home, and you don't want her feeling left out.
But once a day is probably a little much. I remember when I went to sleepaway camp, my (sweet, lovely, eternally well-meaning) mom sent me postcards several times a week. They were carefully worded and didn't make me homesick, but every postcard came with the expectation that I write back, and I really wanted to focus on making new friends and sort of spreading my wings. I (selfish kid, in retrospect) didn't like the sense of obligation that came with all that communication.