How many <account> and <bitcoinaddress> can one client handle in a server environment?
It aught to be able to handle hundreds of accounts and thousands of addresses, although (as far as I know) nobody has done scalability/stress testing with that many (simulated) users.
The listtransactions method scans through all wallet transactions (and then sorts them by time), so if you've got thousands or tens of thousands of accounts with millions of transactions and you're using listtransactions to show customers their account histories, that might be a bottleneck.
However, I wouldn't be at all surprised if something ELSE becomes the bottleneck if you had that many accounts-- for example, there is only one RPC thread handling requests, and some requests (like a send bitcoins request) can take seconds to process. I've been writing code long enough to know that I can't look at code (even code I've written) and figure out where the performance problems are lurking-- you've got to actually measure or simulate real-world behavior.