upperclassmen quota?

There IS a sophomore quota (usually 6-9 I think) and sophomores are considered separately from freshmen, so the freshmen rushing are not your competition in any way - only the other sophomores/upperclassmen who are rushing.

as an upperclassman, you can either do fall informal rush or spring formal rush. very few sororities do fall rush (last year it was just zeta and aopi- aopi because they didn't reach quota and zeta because they're new and under quota. i've heard rumors that tridelt is doing fall rush this year but that could be false so no guarantees on that).
i'd recommend going to fall rush open houses and such, but you're much better off waiting until spring for formal recruitment because you severely limit yourself by just doing rush in the fall. fall is less stressful though so i guess if you fall in love with a house it can work, but rushing in the spring gives you more time to get to know girls in the houses.

for spring rush, sororities look at freshmen and sophomores separately, so as an upperclassmen you will only be compared to other upperclassmen who are rushing. because of that, rushing as an upperclassmen doesn't at all put you at a disadvantage. plenty of people transfer as sophomores, go through formal recruitment, and get into great houses that they're super happy in! good luck!