Hi bludwig,
I'm building a Time Machine backup server for 50 people as well and I'd like to ask you few questions.
1. Are you using Netatalk 2 or 3?
2. Which setup do you have? I have Ubuntu 16.04 and zfs v0.6.5.11-1ubuntu3.
3. Is your server stable with 50 connections? I've heard Netatalk is not scalable.
Thank you 
Br,