There's no limit to the number of serial/UDP commands you can send to or from the BrightSign player. You setup the player to receive serial/UDP commands using BrightAuthor, where you define the serial/UDP strings that are acceptable. The strings can be used to change from one file to another, pause, resume, change volume, send output, etc.
Creating Presentations that accept UDP (Ethernet/Wireless) commands
Controlling Displays (sending serial commands)
Importing a list of Commands and file names as a csv:
This video (based on BrightAuthor 2.0) shows you how you can send commands to playlists created using BrightAuthor.
For new control options available on BrightAuthor versions 2.2 and later watch this video.
Additionally, you can control the BrightSign unit using a script. The link below points to a sample control script and readme. This control script defines a fixed set of playback commands (Play, Stop, Search, Loop, Pause, Resume, etc). Just copy the autorun.brs script and your content to a flash card, and you're ready to send/receive commands. The script also provides feedback when a command is received, when a file is found, when a file loops, etc.
You can also download the files from this link: Brightsign Control Script
Note: For serial commands, BrightAuthor assumes the incoming string is terminated with a carriage return. This can be changed under File > Presentation Properties.