I recently had to cast on 450 stitches for the Linen Stitch Scarf. The pattern suggested, and I definitely agreed, that I place markers every 50 sts. It's a lot easier to count up to 50 nine times than to count up to 450 without being interrupted!
I've kept the markers on as a guide to my progress (I'm a relatively new knitter; learned in May 2011) as I move up/down a row. For example, I know that the stitch right before a marker is either a purl or a knit, depending on which row I'm in. It is not a slip stitch! (At one time it may have been a slip stitch, but not now....) If I end up at a slip stitch right before the marker, I've made a mistake, but I don't need to look further back than the last 50 sts to find it. whew
Then there was the hat class (in July, only seven weeks after my journey began). Not a complicated hat, just a hat in the round needing to be a multiple of 8 sts and close to the right circumference for my own head. The magic number, given my gauge and my yarn (Cascade 220), was 88 sts, the same as the number of keys on a piano. Thus not a difficult number for me to remember. So I cast on and thought I was OK.
I counted the sts twice before advancing to the first round after the CO. I got two different numbers. I asked the instructor to see what number she got. Neither of us hit 88, not one single time. AND...the instructor (let's call her Kathy; the initial at least is the same) pointed out that the CO method I had chosen would not work for a hat, since backwards loop isn't considered very stretchy.
So I pulled it out and started again. This time, however, I used markers to keep the count for me. But where to place them? was the question. You'll probably need to make these decisions, too, once you realize how markers can absolutely save your...ahem...cookies! yes! cookies! in a knitting project.
Every 8 stitches would have been nuts. I'd have been moving markers over so often they would've lost their meaning.
Every 11 stitches would've been meaningless; the pattern worked in multiples of 4 and then 8 stitches.
I wanted a number whose factors included 4 and 8 and which was small enough that I could reliably count it up in the middle of a knitting class with two other students who were blabbing at the top of their voices about everything but knitting, but who then would count OUT LOUD as they were working. "Kathy" did not suggest to them that this was distracting to the others. sigh
I ended up placing the markers every 24th stitch: after 24, 48, and 72, with 16 sts left over at the end. And I did put one at the end, too, to make sure that I didn't add or lose sts on a round. (I seem to have developed a talent for adding sts in the middle of rounds/rows rather than at the ends.)
You'll find LOTS of ways to use markers to help yourself in projects!