[xubuntu] How many home directories for multi boot?

I agree with the recommendation to have separate home partitions for each of the installed systems. (You can also have /home as directories inside the root directories.)

If you want to share personal files (documents, music, multimedia files) I suggest that you have a separate data partition. When you have Windows alongside linux, it is a good idea to use the file system NTFS in the data partition to make it available for both linux and Windows. Otherwise you should use a linux file system (for example ext4).

(Linux can also read and write the Windows [system] partition, but it is risky, it is better to have a separate data partition.)