@molly0xfff as someone who has often underestimated the amount of yarn needed for this sort of cast-on and having to start again, I applaud you!
For those in the dark, to cast-on, you have to start a long way into the ball of yarn and every stitch you add consumes yarn from both directions. Hopefully you left enough length to cast-on all the stitches you wanted, otherwise you have to undo all your work and start again from further along the yarn.