First off- I never get straight-run anything because it has been my (admittedly) limited experience that straight-run always means many unwanted males. Too many boys in the poultry yard equates with an unstable and unruly flock. The constant need for culling keeps the birds on edge and messes with the flock dynamics. I always get sexed birds. You always get a few males accidentally with a large number of females, and that's exactly what I need. I found with straight-run birds I always got more males than females, and that was unacceptable. I hate culling based solely on gender, although that has been necessary in the past. I usually cull because a bird has proven to be unsuitable based on personality (ie. aggressive) or physical traits (ie. angelwing). I have had to cull excess males but it always makes me feel like a brute when having to do so.
With limited housing I would definitely go for sexed birds, but if you still want straight-run then I would order no more than 12. Straight-run is a 50:50 split, so with 12 birds you will get about 6 females out of the bunch. You can cull your males as they get to full size or sooner if you don't care to wait. I don't like wasting birds, so I generally wait until they are 4-6 months old to butcher because then you get a puny, but usable carcass.
I have only given one cull away. She was a great laying hen that was rejected from her her flock after an injury required her to be isolated for months. I have never sold any chickens because I had a bad experience selling an animal to someone once. My culls end up in the freezer. We believe in getting our money back on our birds one way or another, and that's why we butcher them at home and eat them. I can ensure that animals get a clean and quick end that way, which is more than I can say for the animal that I sold who got a bad end that still haunts me to this day.
I hope this helps. Good luck with your new birds. Culling is the hardest part of managing poultry, but it's still worth it to have them around despite it.