The airport express is said to support 50 users, yet in my case (when bridging WiFi to ethernet) it only serves up to 4 clients on the LAN port. A fifth client is not able to obtain an IP address nor ARP requests or anything else. It's completely ignored by the airport express.
So: How many ethernet clients does the airport express support concurrently when bridging?
Note: This applies to bridged mode (ProxySTA) only, where one bridges an existing WiFi (connected to the internet) into an ethernet.