Redmine

1. Maximum number of users: no specific limit, some pages are not optimized for very large groups though (the project welcome page stutters with projects with more than 2k users for example).
2. Maximum number of concurrent requests: Redmine is a rails app, which means each server process is stateless and each server process serves one concurrent request (AFAIK only JRuby is multi-thread-able, and Redmine is not tested/supported against JRuby). Long story short: you'll need one process per concurrent request you want to be able to serve (those can be spread across multiple servers), if you use passenger for example each process will need around 250MB virtual memory.

Shameless plug: I work for finnlabs and we offer various services for and around Redmine, depending on the size of the installation you are planning we might be able to help you with planning, setup and/or operations. There are also other CommercialOfferings if what we provide doesn't fit your needs.

I think the "blocks" plugin can filter out the project member box whoch coul make problems with large sets.