There is a hard cap of 1000 songs (including disc songs) that are available at any one time. This means you can have 917 songs on an SDHC card. The Wii devs explain the reasons for the cap in the thread I linked. The Wii can now use a 32GB sdhc card. It used to be limited to 2GB (4GB with some fiddling) but that limit was changed with a Wii system update. Depending on the size of your songs, you can get in the neighborhood of 850 songs on a 16GB card. If I go to 1000 I'm going to need a 32GB card, but they are cheap and a backup at this point is clearly a good idea.