While the actual numbers will vary... Most SNES VC games will take up around 60 blocks for the game & 1-3 blocks for save data (including interrupt saves).
So using 65 blocks for a round number... You can get about 25 (lowball estimate) on your Wii without having to store them on an SD card.
I will note that you could store up to 240 SNES games (which is currently more than what's currently in the Wii Shopping Channel's catalog) on a 4 GB or larger SD card (costing around $20-$30), as that's the maximum supported by the SD card channel.
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