Since straight run is a great opportunity for hatcheries to dump males, I would guess you'll have a majority of boys. If you are buying a large enough number, some should be female. Do you have a plan B for rehoming if you get a number of boys? Most areas will tolerate hens but not roosters.
Hello and Welcome To BYC! If they are actually straight run/ as hatched they should be about 50/50 male female, but that can vary with any given batch of chicks. If they are just left over chicks it is any ones guess but probably you'll be getting a lot more males. You might want to post on the What Breed Or Gender Is This forum, you probably won't be able to tell until they are a little older. www.backyardchickens.com/f/15/what-breed-or-gender-is-this