Newbie's dumb question about casting-on

[missmo]

When a pattern says to cast-on 200 stitches, do I really make an entire row of 200 stitches? How does it all fit on the needle?? I feel sure that my mathematically-challenged brain is missing something because it's hard for me to get even 40 stitches on my needle, and I'm using the right sized yarn and needle as the instructions call f