Hi Peter,
to my opinion it is a better statement: how many files do you want to save on a SD-card. It depends on the way files are saved and the character of the song. (did you saved the files with all options, or with only sequencer or only the panel memories? How long was the composition? etc.). So, when you save a file, pay attention that you only save the needful options/setting to the SD-card. Otherwise you waste a lot of space on the SD-card. (See page 2 of the save-menu).
Also here it's important to wonder: what files are really needed to save on the SD-card? I don't use my SD-card as a storage for all my styles. That would be an very expensive solution.
Therefore I use my computer/CD-roms.
Be aware of the fact, althought on a 128 MB-card you can make 99 folders, it does not mean that you can fill them all with 20 performances!!
As Joan says it costs less to start with a smaller one to discover if you need a big card (the smaller card could you use anytime later on)
Greetings,
Cees