How many resources does a mumble server need?

CPU consumption should be minimal, for the most part it resides in the network libraries / ip-stack. murmur doesn't do decode-multiplex-reencode. The audio-streams are forwarded/copied to the appropriate clients verbatim.

RAM working set are limited to input/output buffers for the clients (which would be small since one of the project goals is optimizing for low latency), user metadata, some state and acl's.

According to the mumble/murmur FAQ:

Worst case scenario: Number of users × Number of talking users × 60 kbit/s. With less aggressive quality settings, it's ~20 kbit/s, and the bare minimum is 12kbit/s. Note that Mumble is geared towards social gaming; its quality enables people to talk naturally to each other instead of just barking short commands, so the amount of "users talking at the same time" can be somewhat higher than expected.

This means that a server with 20 players and 2 players talking at once requires 0.8-2.4 Mbit/s, depending on quality settings. In the server's .ini file, you can specify the maximum allowed bitrate for users as well as the maximum number of clients to allow.

Disk IO and storage space needed, is going to be minimal, maintaining userdatabase and acl's.

If you want a more authoritative answer you could check out the contact section of the FAQ, this sounds like something that possibly should be included in the FAQ.

From the README:

Bandwidth usage

Mumble will use 10-40 kbit/s outgoing, and the same incoming for each user. So if there are 10 other users on the server with you, your incoming bandwidth requirement will be 100-400 kbit/s if they all talk at the same time.

From the Mumble wiki

Based on data from out testserver, murmur will use about 40 MB of virtual memory, of which > about 4 MB are resident in physical memory.

My reading of the docs and experience running similar servers (Asterisk and ventrilo) lead me to say that a dozen users should run almost unnoticeably on any reasonably modern machine. Bandwidth consumption maxes out at 64kbps/user, RAM and disk are likely fairly minimal, I can't see more than a couple hundred megs of RAM at most (if written very poorly) and likely much less. Disk space should only be the install size + whatever logging you're doing unless you're recording conversations or something.

Your best bet is to try it and take some measurements.