HI arretx, welcome to the Xibo community!
You’re right, you can indeed use a single Player and split the signal to multiple connected screens, as long as you have a splitter device with enough outputs for all of the screens you wish to show the content on. Alternatively you can use a single Player for each screen, allowing you to set content for individual screens. There are other methods of setting up the Xibo for Windows Player, such as Video Walls and multiple instances of the Player on one PC, but these require more specialist equipment.
Raspberry Pi is a platform that is not officially supported, although I am aware that some users have had varying results using them. i would recommend an alternative device for your Players but I appreciate this is of course your choice.
Xibo for webOS has a very specific list of supported devices, below is a link to that list. Whilst I have not tested it, I would not expect it to work on a Raspberry Pi:
List of Supported webOS Displays
I have also included a link to the recommend hardware for Xibo for Android as well. Please note with the Android player that the listed devices are recommendations as opposed to a definitive list like with webOS, but hopefully the spec of these models will help:
Recommended Android Hardware
I hope the above information is helpful, you may also find more advice on the Hardware section of the forum, where many users post their experiences with various devices.