How many NGinx redirects is too many?

A redirect is not really a problem as there's no backend being contacted. Just Nginx. And Just Nginx is really, really fast. As in 10,000+ connections per second fast

You should probably worry more about a cluttered configuration file before you worry about if Nginx can handle it.

As a bonus, I was recently working on a full page caching solution with Nginx and I used Memcached as well, a neat trick to improve performance some is to use the upstream keepalive module to keep your connections open.

It only works with Memcached backends, though, so don't use it with any sort of fastcgi or HTTP proxying.

I'm working on a news centric website thats been around for years and is steadily accruing more and more page redirects

Working on a similar project, I've included an Nginx conf with our 1.8 million accrued redirects on an AWS m1.xlarge (4 CPU, 8 ECU, 15 GB memory).

With 1.8 million redirects, Nginx reloads fine but the response times are of the order of several seconds when put under a load of a single concurrent user.

It seems to perform OK with tens of thousands of redirects.