It isn't officially SDXC compatible, although some are using it by formatting the microSDXC card as FAT32. The Clip+ has a song limit of around 8,000, although some have hit the limit with as low as around 5,000 songs. THe limit is the size of the database, and if you have long tags, heavily nested files, or embedded album art, you could hit the limit with much less than 8,000 songs.
I think Sandisk discourages people from formatting SDXC cards as FAT32. I don't know why.