Xibo itself doesn’t impose a hard limit on the number of players that can connect to a single CMS.
We know of production instances of Xibo running many hundreds of players against a single server. The limitations come from the hardware you choose to deploy your CMS on.
Asking what hardware is required is a little like asking how long the proverbial piece of string is. Everyone’s display workloads are very different depending on the frequency and content of the media updates made to the layouts the CMS is hosting, the frequency with with the displays connect to the CMS and how aggressively media items such as RSS feeds are set to be cached.
For most small to medium deployments, quite modest server hardware should be sufficient. For larger deployments, take the time to simulate some production workloads and then scale out from there to work out the resources you’ll need to service your clients.