It's hard to keep track of how many stitches you have cast on when you're casting on a large number, or to accurately count the number of stitches you have made once you've finished casting on.
A really easy way to help yourself count stitches is to slide a stitch marker on your needle after casting on a certain number of stitches. I like to slip a marker after every 20 stitches.
If you know there are 20 stitches between each marker, it's a lot faster to accurately count how many stitches you have on the needle. When you knit the first row, just take the counters back off the needle as you get to them.