It is not really 50-50, though ordering twice as many chicks as you want pullets is probably wise. Let me give a few examples.
If you order 1 chick, the odds are 1 in 2 you will get a pullet.
If you order two chicks, the odds are 2 in 4 you will get 1 pullet. But the odds are also 1 in 4 yu will get two pullets. They are also 1 in 4 you will get 2 roosters. So the odds are 3 in 4 that you will get at least 1 pullets (sometimes 2) if you order 2 chicks.
If you order 3 chicks, the odds are 1 in 8 that they are all roosters or pullets. So the odds of getting at least 1 pullet out of 3 chicks is 7 in 8. Pretty good odds. The odds of getting at least 2 pullets out of three chicks is 4 out of 8, three times it is 2 pullets and 1 time it is 3 pullets.
If you order four chicks, the odds of getting no pullets is 1 on 16, so the odds of getting at least one pullet is 15 out of 16. The odds of getting 2 or more pullets out of 4 chicks is a phenomenal 11 out of 16. Of course, the odds of getting 2 or more roosters is also 11 out of 16.
Maybe this will help explain it.
4 males - 1 out of 16.
3 males/1 female - 4 out of 16
2 males/2 females - 6 out of 16
1 male/3 females - 4 out of 16
4 females - 1 out of 16
This of course assumes it is true straight run. I've only ordered chicks once and I got 4 roosters and 9 pullets out of my straight run order. If the hatchery is ethical about how they do their straight run, the odds mean something. If they are not ethical, then the odds mean nothing. I ordered once from Cackle and from my results I believe they were ethical. I cannot speak for anyone else. And remember, it is only odds. There will be some orders that are almost all roosters.