I am the eldest of of 3, sort of 4 due to an extra sister acquired at age 11 :)
I have 3 - my middle one was overheard telling his best friend that Mummy had had a brother for him so he'd have somebody to play with, because his big sister is always going off and playing with her friends :) If you parent consciously you can try to ensure they all get equal attention, my parents somewhat expected the oldest to take responsibility for the youngest and that they would have to parent each successive child less I think!
As a child my middle sister and I played together a lot because there weren't many other children about where we lived, but we also fought really bitterly. My main rivalry was with her, and resentment at the fact I was always supposed to rise above taunting from her and not retaliate because she was little (she was only 2 years younger). I never felt anything other than protective towards the youngest, however once I hit my teens I hated always being told I had to take my sister with me when I wanted to go out, and my parents mixing bribery and emotional blackmail to make me take the youngest on holiday with me when I was at uni and she was 15! I aim to avoid doing glaringly ridiculous stuff like that!
We did have to get a bigger car though, my parents used to stuff one of us in the boot, and later had a serious of ridiculous unsafe vehicles to transport us in, including a very ancient agricultural land-rover without seats in the back...
I am glad to have more than one sibling now as it makes visits back to my parents easier if they are around. I used to enjoy the hubub of lots of us as a teen, would never have liked it to be just me and my parents - as a child I think I would have said more were better, but would never have chosen to be the eldest (I always secretly longed for a big brother for some reason).