How many screw in tree steps would it take to get about 15 feet off the ground??

Depends on our stride.. But I would guess about 8

I carry 8 in my pack.

I do it with 5, but I cheat. I put one in as high as I can reach, then another on the opposite side as high up as I can step. I grab the highest and step on the lowest and pull up. My other foot then goes on the first. Then I repeat the process until I get where I want to be. Being 6'5" also helps.

be smart eliminate the screw in treesteps, they can be very dangerous as my cousins friend out a few years ago. He was inserting some steps so he could hang his stand for the season, well about the6 step up, it breaks and he slides down and cathes his leg/groin on the next step and got a nice gash and stitches to go along with it. Please be careful. I recomend Climbing sticks along wiht a good climbing harness.